While looking for something else, I just found these Key Commands under Audio key commands:
Convert Tracks: Mono to Multi-channel
Convert Tracks: Multi-channel to Mono
So the can be run as one-offs or assigned to a Key for repeated use. This splits a stereo into two tracks or combines two mono tracks into a stereo track, or since it’s multi, I assume it can combine and split multi channel (5.1 or 7.1) but I didn’t test that.