background subtraction live camera video

I have two cameras recording two separate videos
I would like to subtract the background in both videos and only leave the people in the video remain. After subtraction, I would like the background of the video to be transparent (like a .PNG pic). I tried a few patches here but all the patches make the background black instead of transparent.
question 1: how to make the background transparent
question 1: once the background in both videos are subtracted and being transparent. How to add the two videos together and make a third video? (i guess i can use jit.op?)