OK, reading his again, I may have misunderstood.
If what you have is two cameras, both with stereo mics, then no, there is no need to fill either channel. My guess is that if you listen to each one individually, the left and the right channels are probably almost identical. SO there is not point to changing that.
I was only saying that it is OK to take one stereo track with two distinct mono mics as input, duplicate the channel to make it proper stereo on each channel. It does not make mono, but rather, identical left and right channels.
In your case, you really don’t need to do that anymore if you have two distinct stereo mics.
Steven
https://www.stevengotz.com