I installed a set of cameras on my bike and the streams are saved separately on the SD card. There is some kind of Windows tool included but I wanted to combine them on Linux, since I don't use Windows at home.
This ffmpeg command will take 2 streams and put them together side to side in a new MP4 file. Make sure both streams have the same resolution!
-i CAM1.MP4 \
-i CAM2.MP4 \
-filter_complex '[0:v]pad=iw*2:ih[int];[int][1:v]overlay=W/2:0[vid]' \
-map [vid] \
-c:v libx264 \
-crf 23 \
-preset veryfast \