Looks like I found the answer from PG to a similar question I asked some time ago.
In short (if anyone else has this question… or for my future reference when I forget it later!), set it up as a 4-channel surround sound montage (or multi-mono) in File Info → Channel Configuration. Then enable the Master Section “Mix to Stereo” option for playback (if you only have a 2-channel playback system).