ffmpeg -y -f video4linux2 -vcodec mjpeg -s 1280x720 -r 30 -i /dev/video1 -f video4linux2 -vcodec mjpeg -s 1280x720 -r 30 -i /dev/video2 \
-filter_complex 'pad=iw*2:ih[src]; [src]overlay=w' \
-vcodec copy -an -ar 44100 left.avi