ffmpeg - Overlay one video on to another
The command below resizes the upper video to match the base video and also gives it transparency.
ffmpeg -y -i /sdcard/videokit/two.mp4 -i /sdcard/videokit/inone.mp4 -filter_complex "[1:v][0:v]scale2ref[ua][b];[ua]setsar=1,format=yuva444p,colorchannelmixer=aa=0.5[u];[b][u]overlay=eof_action=pass[v]" -map [v] /sdcard/videokit/output.mp4
The scale2ref resizes the second video to match the dimensions of the first. Then the colorchannelmixer tweaks the alpha channel to give it 50% transparency.