Back in the day, the day being sometime in 2003, we watched "Breakfast with the Arts", a show on A&E hosted, at that time, by Elliott Forrest, a very talented and knowledgeable presenter and interviewer.
Marianna Rosett and Ken Bichel. They asked for themes, a set of them, and they then wove a piece of four-handed music right there and right then.
Sometimes it was like ping pong or badminton, as they traded musical phrases back and forth from piano to piano. Other times - waltzing, but trading the leader's role back and forth. I'd never heard anything like it before. And haven't since.
We found a CD of theirs and I'm glad I bought it. It doesn't seem to be available any longer. There does seem to be a download of the music available here.