What audio editors do you have?
- Sound Forge
- all of the above
The method varies per program.
I use Sound Forge
for something like this. Load the WAV file (convert to WAV if it's something else!), then double click the good channel, CTRL-C to copy, then double click the 'bad" or empty channel, CTRL-V to paste. Takes a few minutes to paste, and you're done. Save the WAV. Convert back to AC3 or MP2 (or whatever) and you're good to go.