./ffmpeg -i 1.mp4 -i 2.mp4 -filter_complex \
" \
    [0:0]scale=320:200[v1], \
    [1:0]scale=320:200[v2], \
    [v1][0:1][v2][1:1]concat=n=2:v=1:a=1[v][a] \
" \
-map "[v]" -map "[a]" 3.mp4