This is a request. They asked for some SolKat. I didn't know if you wanted human or troll. Or what kind of anatomy you wanted for trolls (tentacles, nooks, etc.) so I'm just gonna roll with it again. This is most likely PWP (plot? What plot?) cause I honestly cannot think of a freaking idea for this thing.
I absolutely adore SolKat so I wouldn't mind some more requests for them! Just please give me a prompt, a rating, a ship and if you want Humanstuck or regular old trolls. Mkay? Easy enough.
Alrighty, here goes nothing.

Sollux smirked down at the writhing body below him. His matesprit was sprawled out on the floor, his hands tied to the leg of the desk and his legs splayed wide open. A whine escaped his lips though he would deny it later to save his pride. Sol loved his darling matesprit more than anything but his dual personalities liked to mix things up from time to time. He often switched between viewing him as a matesprit and kismesis. The other didn't seem to mind as he always had this obsession with keeping a lover all to himself and filling all of their quadrants. Even as he glared up at red and blue eyes deep down he enjoyed being teased and driven to near madness.
Red flushed his body visibly below grey skin. His bulge, completely unsheathed now, curled in on itself in an attempt to create some sort of friction. His mouth hung slightly open as quiet noises slipped out of him. Sollux leaned over him placing a hand on either side of his head and grinned down at the boy.
"What'th wrong KK? You look uncomfortable." He teased, acting as though he was concerned for the smaller troll.
Karkat growled up at him though it was rather weak sounding. "You're a fucking tease."
"What? Me?" He chuckled. "Maybe if you athk nithly I'll give you want you want."
"Fuck you."
"Don't you mean 'fuck me'?"
Sollux laughed when Karkat groaned sounding rather pathetic and desperate to be touched. Careful of his claws, he stroked one finger down the length of Karkat's bulge dragging a moan out of him. He quickly retreated again when the tendril tried to tangle around his fingers. Diluted red material dripped from Karkat's bulge and nook and onto the previously clean floor.
Feeling rather impatient Karkat decided to take matters into his own hands. Well, not literally his hands as they were still tied above his head. His legs, however, were not tied down. He put on a face that would make him look needy and pathetic to catch his matesprit's attention.
"If you're not going to touch me, can you at least give me a kiss?" He asked, keeping a frustrated tone to his voice.
Smirking, Sollux leaned over him and pressed their mouths together instantly tangling his split tongue with the other. While he was distracted by the other tongue licking patterns across the roof of his mouth, Karkat bent his knee and maneuvered his leg until it was between Sollux's. He then raised his leg, pressing his thigh against his two bulges and his nook. Sol moaned into the kiss as Karkat continued to grind his soft thigh against the other boy. It appeared his plan was successful as both of Sollux's bulges tangled around Karkat's single one making him buck his hips. His thigh ceased its movements as he instead began to grind his hips up against Sollux. He lowered his bottom half to press their bulges closer together as his tongue continued to explore Karkat's mouth.
They pulled their lips apart, needing to catch their breath. Instead Sol began kissing and licking down his jaw, neck and shoulder.
"Sol." He whined, dragging out the name. "More."
Sollux continued his work leaving marks across his shoulders, chest and neck as one hand slipped between them, past their writhing bulges and to the opening of Karkat's nook. Two fingers traced the outside of the wet nook causing a strangled gasp to leave his lover. His claws were much to sharp for him to push a finger inside. Instead he continued to pet the outside with two fingers.
"Enough already." Karkat sighed. "Hurry up and do it."
Sollux chuckled against his neck as he untangled one of his bulges and began to press it into the soft, warm comfort of Karkat's nook. The smaller troll pulled at the ropes confining his wrists, dragging the desk closer to them, as he was completely filled. Sollux wasted no time thrusting the rest of the way in earning a shriek of pleasure from his matesprit. As one bulge pressed in and out of the other's nook, the other rubbed against Karkat's own bulge. There was definitely a benefit to having a lover with two appendages rather than one.
Arms held the other close together as each thrust was met half way and moans and grunts of pure pleasure filled the room. Sollux's bulge pressed deeper inside the other, wriggling around as he thrust in sending a tingle up Karkat's spine. His back arched off the floor, nails digging into the rope enough to snap a few fibers, and toes curled into the other's thighs as Karkat released genetic material from both his bulge and nook. Sollux was close behind, yellow mixing with red to drip orange onto the floor between them. They collapsed together, panting and groaning as their over sensitive tentacles brushed against each other.
Sollux reached up to untie the rope, grey arms coming around his shoulders instantaneously. He held the smaller troll close to his chest as he rolled onto this back, pulling Karkat with him. They lay together for a long moment, neither able to move. When their bodies were functioning properly again they rose only to climb into their shared recuperacoon, slipping into the warm sopor. They held each other close as sleep claimed them.

A/N may contain spoilers depending on how far you have read!

I really like the idea of Sollux finding a kismesis and a matesprit in one person. This was mostly brought on by the "Attempt rare and highly dangerous 5x SHOWDOWN COMBO" page when Sol was staring at Gamzee and switching between a heart and a spade rapidly. As for Karkat, I believe that the descendants inherit some of their ancestor's traits. Such as the friendship between Sol and Karkat reflecting the relationship Sufferer had with Psiioniic. Get it? Anyway, The Sufferer had The Disciple in all 4 quadrants, which is why Karkat attempted this with Terezi. It makes sense to me that he'd attempt this with all possible love interests, right? And Sollux seems like just the right troll to put up with this.
Also, in Karkat's a Sollux's first conversation seen in the actual story it seems like they're hate flirting a little bit.
Anyway, you guys don't care about this. You came for the smut. First time writing troll on troll. Send some prompts and I'll write more! From now on all requests that come without a prompt will just be PWP.