I didn't think I would switch back and forth between the two paintings in my diptych as much as I have. I find I'm almost doing them in tandem. I knew I'd want to do both faces first, because for me, I have to get the faces right before anything else comes together. It is not fun to spend time and effort nearly completing an entire painting, and then crash and burn because I saved a face for last and just can't get it right. It has been a long time since I've had a total fail on a figures face but I always feel that possibility is there.
A diptych, as I would describe, is basically one scene that is comprised of two paintings. The paintings share a common edge. Originally the term defined them as being displayed hinged together, but today they are commonly just hung side by side. The two paintings can also work by themselves.