I think it's possible ... but not easy or common.

My ex and I are civil to each other. We decided long ago that was the only way to be, since we have a child together. I suppose you could even say we are "friendly" with each other ... we chit chat for a few minutes about non-crucial things if we call each other about our son. We give each other a hug goodbye when we meet to pick up or drop off our son. But are we friends, in the true sense of the word? I'm not sure. We caused each other a lot of heartache and pain. We obviously could not live together. However, we shared everything for over a decade, including the most priceless thing of all, our child. I think, if I were truly in serious trouble, I could call Greg and he would help me ... and not just because I'm his son's mother. I know I would do the same for him. So, I suppose that makes us friends ... but not even close to the friendship I have with my "real" friends now. I think it's just more of a long, shared past and a child together.