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