For me, it came like a bolt out of the blue - a match made in cyberspace.
Kat Flannery wanted to write twin, historical romance novellas, set at Christmas time and she invited me to work with her. Since we were writing twin novellas, I suggested twin sisters as the connecting characters. This was because I have twin cousins whose lives have had parallel events in their lives. (Both of them married men named Leslie, for one. This was very confusing since my cousins' names are Lesley and Hilary.)
I don't remember which one of us came up with the Civil War as a setting, but there was a symmetry to the premise. Twin sisters, separated by war. One with a southern family. One with a northern family.
I've haven't collaborated on a work of fiction like this before. There were time when it was a bit like a game. One of us would come up with something about the twins and their family, then the other would find a way of incorporating the detail. Promoting with a partner is a lot more fun too. We each have our strengths in this area and that works out for the best.
I have a couple of other projects waiting my attention but, when I'm caught up. I look forward to new adventures in partnership.