Oh, gosh, what I meant by separation was more of a physical distance between them - but either way works! If you're interested in the letters angle, it might be interesting if the whole adultery thing started out as just an innocent correspondence? (Except not in, you know, a You've Got Mail sort of way...)
Since I've been really interested in language issues, too, there's always the idea of them meeting because one was in the other's country for business or pleasure and used the other as a language partner or tour guide or something? But - I'm sorry, I'm just of just gushing a million things at once. I could set up a filtered post, if you'd like!