By Channel A and Channel B, do you mean the left and right channels?
If so, you can easily apply the audio from the left channel to the right or the right to the left using the Fill with Left or Fill with Right effects in the Audio Effects. How you get to the Audio Effects depends on which version of the program you're using.
Interesting... I go to Audio Effects and drag Fill with Left onto my clip... no change in audio - both right and left play as before.
I go to Audio Effects and drag Fill with Right onto my clip..no change in audio - both right and left play as before.
Without seeing your timeline and the waveform on your clips, it's hard to make any further recommendations.
Meantime, when you say "right and left play as before", you're judging by what you see in the audio meters, right? Not by what you hear out of your speakers, right?