ffmpeg -i left.mp4 -i right.mp4 -filter_complex "[0:v]crop=638:720, pad=640:720:0:0:green[tmp0]; [1:v]crop=638:720, pad=640:720:2:0:green[tmp1]; [tmp0][tmp1]hstack[v]; anullsrc[silent1]" -map [v] -map [silent1] -ac 2 -shortest -y o.mp4