You'll get different suggestions/methods, but what I use is the FILL LEFT and FILL RIGHT audio effects. If you want just the mic on the left channel of a stereo clip, add FILL LEFT and then you will only get the left channel audio, on BOTH the L and R channels.
To expand on that, you could copy/paste to duplicate the clip, and add LEFT to one clip and RIGHT to the other, and then mix them as if they were seperate channels to start with. Or, add razor cuts to single audio clip at points where you want to switch mics L or R, then alternately apply the effect as needed to each segment and add audio crossfade at cuts. Many ways to work with it.
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