I don't have or know premiere, so apologies if I've got this wrong, but from what I understand you have a left audio channel with one voice on it and a right channel with the other voice on it, and you want to copy the left channel to the right for certain sections and vice versa?
Assuming that you have razored in cut points to define where you wanted to do your 'fill left/right' if you were in premiere, can you not just separate these channels from being a stereo pair (with alt-L), centre the pan for the clips (with ctrl-.), then delete the channel you don't want for each section?
You can select the entire interview at once to separate the channels and centre the pan, so depending on the length of interview it shouldn't take any/much longer to delete the channel you don't need for each section, and the results should be comparable. i.e. a mono track with the pan centred plays the same through both left and right speakers.
Not quite what you wanted, but I hope this is of some use...