If the two channels are identical, and even if they aren’t, you can always just use one of them.  If you have truly stereo audio, then you have to decide how you want to monoize it.  Usually just averaging the channels works OK, but it could be problematic especially if the two channels might be out of phase, which can happen due to processing or just because the microphones were far away from each other.  There are probably more sophisticated techniques than averaging that could be useful in some cases, but I never tried them.