ffmpeg -i grab1.mkv -i grab2.mkv -filter_complex '[0:1][0:2] amerge, pan=stereo:c0<1.3*c0+0.1*c2:c1<1.3*c1+0.1*c3 [a1]; [0:0] [a1] [1:0] [1:1] concat=n=2:v=1:a=1 [vout] [aout]' -map [vout] -map [aout] -c:v libx264 -crf:v 28 -pix_fmt yuv420p -profile:v high -g:v 500 -preset:v placebo -tune:v animation -c:a libfdk_aac -profile:a aac_he -b:a 48k -movflags +faststart out.mp4