What I was trying to do was to use the left channel when it was cleanest (it was a PA output) and the right channel when it was cleanest (it was a live mic). I was either using one or the other, never both.
I have since restarted my project, made a duplicate audio track, made one track “left only” and the other track “right only”.
Seems to be a solution, though not a very straightforward one.
Thanks again,
Robert