#mix to file live PIP:
melt A.mp4 -filter watermark:B.mp4 composite.geometry=10%,10%,20%,20%  

#live webcam stream PIP:
melt A.mp4 -filter watermark:video4linux2:/dev/video0?width=320\&height=240\&frame_rate=15 composite.geometry=10%,10%,20%,20%

#audio from both videos
melt A.mp4 -filter watermark:B.avi composite.geometry=10%,10%,20%,20% -audio-track B.avi -transition mix