Wow. I suspect if our others separated, we'd be friends, but not much more than that. One of the reasons it works is that there's someone special for both of us, and I think they would feel the same way, so the number one thing we'd have to deal with is the loss... As for how important the health of the relationship is, I'd say it's absolutely crucial to functioning as a quad. We've seen all too clearly how one couple's problems ours or theirs (especially in reference to communication and intimacy levels) can negatively impact the whole. In short, if you're not good in your primary relationship, then you're in a bad place to deal with the challenges or even the joys of the secondary. Your heart simply isn't in it, and that's painful to the other people who love you. But, you know, maybe I think this way because our relationship is long distance. Maybe it's different when you're consistently present. Perhaps this is why quads are so challenging. A successful marriage/relationship takes work. How much more so a relationship that consists of two distinct marriages/relationships?
It will be interesting to read what others have to say.