In that case....I think I would go back and forth but, and this is extremeally important, IN LARGE CHUNKS. I don't think it's good to go back and forth, a few sentences here, then here, or even in a few paragraphs, in chunks. Make it clear they are simultaneos. If you do do it chapter here and then chapter ther, make it clear to the audience that they happened at the same time. Brains tend to follow chapters as if they are chronological unless specifically informed that they are not chronological. Then again....I could be wrong. Suspense, all that other stuff is also important. It depends. Only the author (you) will be able to discover the actual way to do it. Try reading it as if you have never read it before, from the readers PoV, not as a critic or a writer. Then sit back and say, "that was a good story" or " that sucked" or "WTF was going on?"