ffmpeg -i sample-a.mp4 -vf "scale=640:360:force_original_aspect_ratio=decrease,pad=640:360:(ow-iw)/2:(oh-ih)/2,fps=30,setsar=1" -an -c:v libx264 -pix_fmt yuv420p norm-a.mp4
ffmpeg -i sample-b.mp4 -vf "scale=640:360:force_original_aspect_ratio=decrease,pad=640:360:(ow-iw)/2:(oh-ih)/2,fps=30,setsar=1" -an -c:v libx264 -pix_fmt yuv420p norm-b.mp4
ffmpeg -f concat -safe 1 -i concat.txt -c copy joined.mp4