I do love me some Kyman somno. There is an absolutely fantastic fic with this premise by wizardmonex (https://archiveofourown.org/works/52071187) so check that out if you haven't yet. Cartman has mastered sneaking into Kyle's room after many years of experience. I think the first few times Cartman rapes him, he is totally unaware. One night, he wakes up on the brink of an orgasm. Sussing out the situation with his eyes still closed, he pretends to be asleep while Eric finishes them both off. He's had time to learn how Kyle's pussy works, meanwhile, Kyle is thinking he's just had some really good dreams lately.