In your mixer window – pan the pre-recorded track(s) to the
left and pan the track your recording on to the right (or
vice versa).
You’ll end up with a WAV which has a mix of the two tracks
on its left (or right) channel, and just the newly recorded
material on the other. There are then a couple of ways of
what you do next, but the basic way is to ‘silence’ the
mixed channel and then copy and paste the recorded material
onto it to make it stereo. Alternatively, once you’ve silenced
the mixed channel you could hit F11 and convert to mono and then
convert back to stereo again.
D.
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