Fred and George Weasley were always next to each other, side by side. If one of them did something the other had to be there. It blurred the lines of who was who, who felt what. They felt what the other felt always. In complete darkness they knew that the other was there, and when they feel that same desire, they know they have eachother.
Though that feeling is a major part of it, it was also love, pure, unconditional love. Something that no one but them can understand. Of course inbreeding was common amongst pureblood families like them, there was no point for brothers to be doing that, let alone twins. They can't exactly have kids, it's just unnecessary. But to them it was far more than just keeping the family line going. They love eachother, everything about the other, body, face, voice. The one they share, so glad they share.
They can't help the minor euphoria that goes through them when they get mistaken for each other, it was a strange joy. They didn't understand it. Perhaps it was the reminder that they are almost identical to the person they find so attractive and beautiful, maybe it was just that they love the others name. But more than anything? It was that they felt like one once again. It was everything, all of their fulfillment needs, pleasure needs, and important needs. And on top of that, they love each other's personality. Although they had similar interests and most can't tell their personalities apart at all, they can. Fred was louder, cares less, rougher. George was more sympathetic, quiet, sweet. If Fred does something a little too extreme, George is always there to balance it out, let him know that it's too far and give him better ideas. They depend on eachother.
They need eachother. It was no surprise that they couldn't stop love when it first sprouted, they buried it but the flower grew more and more and peaked through the dirt, so eventually, they moved it into a pot, protected it and watched as it pollinated the surrounding area and as it grow into a beautiful garden. Their love was powerful and strong and unwavering, no one can separate them. They'd either be together forever or for the rest of their lives.
When they're alone, finally have a moment to themselves, finally can feel skin to skin, body to body — god it's heavenly. They could live like this, wrapped up together, Freds left hand on George's hip as he looks down at him, the other on his cheek. Quiet noises, his voice, no, their voice, smooth and soft, let out a moan as he sinks in once again, over and over again. In this moment they can't remember whose name is who, who's doing what. They are basically eachother.
Fred-… George-?… no, Fred knows exactly what George likes, because that's what he likes. No tip teasing, just deep thrusts. Still gentle of course, he couldn't hurt him if he was given 300 thousand galleons, but he made sure he was still going deep enough to make George whine. Fred pulls his spare hand off the others cheek, moving it to occupy the free side of his hip. He burries his face in George's shoulder as he adjusts to move deeper, finally hitting his prostate. Quiet unneeded whispers of 'I love you' amongst jokes and sweet words as they meet.
God every movement felt like heaven on earth, precum smearing everywhere. It's hard not to cry and laugh and everything when doing something like this, it brings out those complex emotions in tenfold. George whines and whines as the peak nears and all at once he practically blacks out for a few seconds, cum all over his stomach and more in his ass, and they are one once again, just like the first time. They don't know who is who, who did what, they are a single being, united, and they never want to be separate people again.