Mixing video audio channels

Downmix one video stereo channel to a mono track

ffmpeg -i stereo.mp4 -map_channel 0.1.0 -c:v copy left-channel-mono.mp4
ffmpeg -i stereo.mp4 -map_channel 0.1.1 -c:v copy right-channel-mono.mp4

Mix one video mono channel to a stereo track

ffmpeg -i mono.mp4 -map_channel 0.1.0 -map_channel 0.1.0 -c:v copy stereo.mp4