I was also confused at first but I think the one Ivan's talking about is Till(when he said he looked like someone he knew). His(Ivan) Till and the one he slept with are the same person, though I don't know why he can't recognize him?? Fictional logic probably. That's why Till looked sad when he went out of the room because he knew Ivan loves him, and that he resorted to sleeping with stranger(which is Till) because of his(Ivan) unrequited love to Till.
The Ivan from Rock AU met with Till from Actor AU. Till(actor) and Ivan(actor) have a mutual feelings but won't confess to each other. Till(actor) slept with Ivan(rock) because he wanted to know what it feels like to be held by Ivan(actor)??? And he felt pathetic doing that. Ivan(rock) and Till(rock) have physical relationship and they live together. Till(rock) isn't answering Ivan(rock), and won't show up. I guess we can assume that Ivan(rock) is kind of a fuck boy?? So yeah, Till and Ivan from different AU's met and fucked.

I'm confused. The summary says the stranger is Ivan in disguise, but aren't there supposed to be four people? Am I misunderstanding something, or is the summary mixing things up? Can someone explain, please?