It's hard to imagine them having a relationship as utterly exclusive and wrapped up in each other as they did when they were kids, because though I think that's exactly what Theo wants in his heart of hearts, it's not what Boris wants. Boris only did that for a while because his life was so isolated; when he has opportunities for a larger life, he takes them.

I could see them having a "don't ask/don't tell" relationship like you describe though. It would be very intense and exclusive in short bursts, and then Boris would vanish for a while and do other things but always come back. I think Theo's denial could cope with that so long as Boris makes sure anyone else he fucks around with is never in Theo's vicinity.