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@Baa Yeah it's a little bit complicated. First you have to ensure they use the same codec and that they have the same parameters. If not, you have to reconvert them using complex filters first, and it's a bit messy. Just tested it with two video files, had to do this:
ffmpeg -i 2.mp4 -i 3.mp4 -filter_complex \ "[0✌️0] scale=320x210,setdar=16/9 [ch1]; [1✌️0] scale=320x210,setdar=16/9 [ch2]; [ch1][0🅰0][ch2][1🅰0]concat=n=2:v=1:a=1[outv][outa]" \ -map "[outv]" -map "[outa]" -r 30 concat.mp4