Disclaimer: Ouran High School Host Club and all of its characters belong to Bisco Hatori; this is just a shameless work that isn't for profit.
Warnings and Spoilers: The obvious twincest and possibles sexual situations warnings are here for future chapters. There's also going to be some events built strongly off of actions that are generally OOC for some of the characters for a little while. Spoilers-wise, if you haven't read from about chapter 45 through to the current chapter of the manga, there'll be spoilers for you. Most especially, there will be content from chapters 45, 51, 52, and 53.
Notes: This one, I'm not entirely positive where it came from at first, but I do know exactly where I'm going with it and just where it's going to end up when all is said and done, so at least that's a bonus. Essentially, this came out of rereading the manga again and taking a look at some of what was said between the characters, as well as some of the things that have appeared in certain chapters. Specifically, taking Hikaru's threat of, "If you ever lie to me again, I won't ever forgive you," which was used as his means of trying to get Kaoru to just be honest instead of trying to hide things from him, there formed the question of, "What if he was absolutely serious about it?" Providing that Hikaru actually allows himself to be stubborn enough to follow through with it, this very threat could really spell trouble shortly down the road from there. Another bit that I picked out of the manga was in chapters 67 and 68, in which the twins have made a replica of the Bocca della Verità (you would probably recognize the name as the Mouth of the Truth). Although it is more than just mildly plausible that the twins themselves have, in fact, seen the real thing in Rome as well as would know the history behind the sculpture, I also used this as a gateway for the possibility of them having watched Roman Holiday in their childhood.
I would also like to note here that, while this entire scenario is based off of actions that are fairly far removed from how the twins would react, I am going to try to keep them as true to themselves as the situation allows for. For Kaoru, this will likely be marginally easier than for Hikaru, at least early in; there will be a change in Hikaru that places him more in line with his natural characterization after a couple of chapters.
One final mention is regarding something that is going to probably come off awkward as you're reading the story (more for later chapters, but I'd rather get it out of the way now); as a personal rule, I avoid using random Japanese words and phrases in my writing, with the sole exception of honorifics (because English equivalents to Japanese honorifics don't hold quite the same meaning to them). Proper nouns, of course, always get their name in the proper language (which is why I, for instance, kept Bocca della Verità in Italian). However, there is a point where Kaoru reflects on his own speech patterns in contrast to Hikaru's, and I will mention something about that here (these are, in fact, a tie-in to the first difference between them that the series notes, as pointed out by Haruhi). Kaoru, when referring to himself, will always use the personal pronoun "boku" (and when the two of them are chorusing each other, they use "bokura"), and I've very seldom seen him use any personal pronoun for the word "you" (he'll, instead, generally always say the name of the person who he is speaking to). I've seen him, while talking alone, use "anta" to denote "you" only a very small handful of times, and he will use "omae" if and only if he is chorusing a phrase with Hikaru. On the other hand, Hikaru is pretty consistent in using "omae" as his way of referencing "you," but his personal pronoun for himself changes depending on the circumstance (and, seemingly, on how he's feeling as he's speaking any given sentence; there are many instances of him switching which word he uses from one sentence to the next). He does, in general, use "boku," just like his brother. In fact, he especially uses it when calm, unsure, or joking around. On the other hand, he has instances of switching off to "ore" (and "orera" to denote "we"), as well, and these are usually when he is angry, flustered, irritated, or otherwise worked up. At least, this is from what I've observed within the manga, via close scrutiny of the panels. The other major difference in their speech patterns comes in their general sentence structure, and this is a little more difficult to really explain. I suppose that the simplest method of explaining this is that they simply piece their sentences together a little differently from each other. Usually, it's just marginal differences, but it is there.

A Scattered Dream
Prologue: A Memory of Our Far-Off Past
by Josie (artes)

They'd gotten the idea from an old movie that they'd watched; their nanny had grown fed up with their antics - neither could fathom why, as they were simply being the same darling and perfect saints that they knew they always were - and left them to their own devices in front of the television. As long as neither ended up hurt, she wasn't likely to get in trouble, anyway, so she was certain that there was no harm in it.

The two had struggled against each other, each grabbing for the same remote control to find something to entertain themselves with. The conflict led to a small bout of wrestling, five-year-old bodies rolling over, each holding the other down - Hikaru would later puff up his chest and boast that he had pinned Kaoru twice more than Kaoru had managed to do to him - before one (Kaoru) finally managed to roll on top of the other and, having grown weary of the tussle, simply rolled right back off of him, dusting himself off before turning away haughtily and making his way over to a cabinet. Golden eyes followed his movements as Hikaru called out for him to return, but he merely ignored the words and continued to search, emerging victorious with a second control in his hand; he held it as though it were the world's most prized trophy before pushing the cabinet door shut and returning to his brother's side, free hand slipping into the one that his twin held out expectantly to him. A light squeeze, and then each took turns changing channels.

Finally, they found something that seemed oddly fascinating to them, although neither could tell just why. Perhaps it was the lack of color, or maybe it was merely the seeming simplicity of it. Whatever it was, the two boys were enraptured and managed to stay subdued for the two hours that the film ran. Asked after that first viewing, neither could recall the name of the movie that they had watched while sprawled belly-down on the floor, feet kicking in the air and hand-in-hand.

Years later, they would come to know it as Roman Holiday.

If anyone had bothered to look into the room - no one did, not even the nanny - as the two remained with their eyes transfixed on the screen, they might have noticed that only one laughed when Joe Bradley played his prank on 'Anya' as they stood before the Bocca della Verità at the Basilica di Santa Maria in Cosmedin; the other had flinched. They may also have noticed the curious tilting of identical heads as the runaway princess recounted her dream of a life free of her responsibilities.

When the credits rolled, the two had scrambled up from the floor, amber eyes meeting and silent plans forming between them before grabbing hold of each others hands and racing off to a different room. Not a single soul within the estate seemed to take notice of their shenanigans, whether out of fear for personal sanity or for having grown accustomed to the sight. One reached out to a doorknob and pushed the door open, the other peeked into the room, head turning this way and that before he nodded to his brother and both sneaked into the room as stealthily as they could manage - the staff of the Hitachiin estate would argue that the two young boys could put even the most well-trained spies to shame if they set their minds to it, and only a handful would be untruthful in the statement - shutting the door soundlessly behind them.

They only broke apart from one another after having taken a long look around the room, each running to pick up objects, toys, and all kinds of knickknacks from shelves and closets and chests, amassing a fair pile on the floor before sitting in front of it, eyes of mirrored gold mischief forging rough ideas and blueprints before they moved again, perfectly synchronized as they placed piece after piece, item after item. A base was formed, and then sides lifting up, farther and farther. Here there was a gap, just large enough for tiny hands to pass through, and over there were impressions that may have been eyes. When there work was done, their masterpiece complete, the two identical little boys sat holding each other's hands, staring up at their project with glowing pride and admiration. One boy squeezed the hand of his brother, and both turned their heads to look at each other, a silent question held in their eyes before the first grinned, reaching his free hand towards the opening. The 'mouth' of their current beloved work. "I'm gonna do it."

One twin's head tilted to the side in confusion and concern, lips pursing and eyebrows drawing together; he knew what they had created, it was right out of the movie that they had just watched together. And if it was that face, then putting his hand into that mouth would be dangerous. Especially since they had just lied to their nanny about what they hadn't put into her breakfast earlier that day. "But Hikaru..."

"It's fine, Kaoru," his brother reassured, raising his head with a slight swish of short, copper hair and a childishly overconfident smile, perfect and gleaming white teeth showing widely; he made a fist with his free hand and raised it into the air triumphantly. However, his twin tugged on the hand that he still grasped, frowning and eying their creation with apprehension as he bit his lip.

"But... but, Hikaru, it's supposed to take off the hand of liars..."

Kaoru's voice was unsteady as he'd spoken the words; he looked into his brother's eyes, gaze steady and horribly worried as he squeezed onto the hand held fast in his own. It was as though he was perfectly confident that his words were simply the most logical in the entire world, and he couldn't possibly fathom why his older brother couldn't see this. But the amber eyes that he stared into simply rolled in annoyance at his hesitation as Hikaru lowered his hand and reached it towards the 'mouth' again, shrugging his shoulders and setting his lips into a grim line. "So?"

"We-"

And that was when it clicked; even though it was just a game, no different than any other that they'd ever done, the idea of what they were doing had weighed in on Kaoru as he recalled the scene that they had taken it from. He was concerned that something might happen to his twin brother, in light of the harmless fibs that the two always told to them. "I don't lie to you, so it'll be fine," he reasoned with mild youthful exasperation. He raised his hand again, index finger pointing up to further illustrate the excessively reasonable point - in his mind, at least - that he was making. "It's not like anyone else matters, anyway, so it won't care if I lie to them."

Honey eyes lit up at the words and a bright smile broke out on his younger twin's face; Kaoru bounced merrily upon the balls of his feet for a few moments with joy as the words hit him. "Oh. Oh, right!" He continued to watch as his elder twin reached for the third time towards their makeshift Mouth of Truth, gaze transfixed; his body was wracked with apprehensive tingles and shivers as Hikaru's hand disappeared into the hodgepodge of objects they had pieced together, fingers tightening as he gripped the other boy's hand far more tightly than he had previously. Suddenly, one of the elder twin's eyes clenched shut as though in pain and he let out a small sound of discomfort. Immediately, his younger brother's eyes widened and, frightened, he began to exclaim breathlessly, "Hika-!" He cut himself off, however, seemingly fearful that he would cause trouble if he completed his brother's name, and the older of the two withdrew the appendage from its place, relief flooding his twin's features before two pairs of golden eyes fell to where his hand should have been. Hikaru's eyes widened first in horrified shock, and Kaoru's followed a split second later as he screamed, "Hikaru!" The sound reverberated in the room as the two continued to stare, before Hikaru pushed his hand out from his sleeve, laughing splendidly at having perfectly executed the movie's prank. The younger twin immediately turned to him, free hand hitting him repeatedly - in the chest, in the arm, once or twice on his face and neck as Kaoru swung wildly - tears welling in his eyes and spilling unabashedly down cheeks that quickly turned blotchy and red. "It's not funny! It's not funny, Hikaru!" he cried, over and over again.

"It's fine, it's fine. Look, look!" Hikaru gasped out between chortles, free hand waving in Kaoru's face, but the action merely caused the other's tears to increase as Kaoru fell against him in a sobbing mess. "Geez, I can't believe you fell for it, anyway; isn't it just like the movie? Come on, stop that."

"But... but you said it wouldn't do anything 'cause you don't lie to me, and then you did that, and... and that meant you must've lied to me if it took your hand," he whimpered pathetically, clinging to his older brother. His tears refused to subside; finally Hikaru began to comprehend what the problem was.

"Kao-"

He was cut off when Kaoru continued to speak. "A-and you're not supposed to lie to me, 'cause you're my twin..."

And there it was.

There was no hesitation as short arms wrapped around a tiny body that was a perfect double of his own, one hand patting copper hair as the other rubbed his brother's back; he shushed Kaoru gently, standing up on his toes to press a kiss to his fretting twin's forehead before smiling reassuringly, but to no avail. Still the tears fell from golden eyes that were hidden from how his eyelids had clenched so tightly shut. Hikaru frowned in consternation, leaning in again to kiss Kaoru's left eye, hands resuming their previous motions as he spoke placatingly, "Kaoru... Kaoru, I've never lied to you, I promise. I promise, I won't ever, ever lie to you. So don't cry anymore, alright?"