I think that John pointed you to, and you found, the PrE forum. Just in case, the method will depend.
Do you have one good channel of stereo? Is the other channel good, but empty, or is there noise in it?
If the former, look into the Audio Effect>Fill Left & Fill Right (choose what you need).
In the case of the latter, I'd edit first in an Audio editing program, like Audacity. There are many ways to get the Audio from your AV file into Audacity, but Importing just the Audio is probably the easiest - basically ripping the Audio from the muxed AV file. I believe that Audactiy can do this. I use Adobe Audition, and it does it easily.
Next, you can delete the bad channel, and dupe the good channel, placing it on the other channel. Save_As 48KHz 16-bit PCM/WAV and Import into PrE.
BTW - PrE will combine all Audio onto the Master Track at Export.