Well if you do it behind a blue screen(you can use a wall,but has to be one solid color) you would use the eye dropper to select that color and then use chroma keyer( not sure what its called in adobe, perhaps generate mask or something) to select that wall. It will replace the wall with the other video but leave your head there. The problem is your not going to be able to mimic the other person head perfectly so you will need to use track motioning( again not sure in adobe) to play your head in the right spot. You may be able to get away with just changing the spot of the video (x,y, angle) with ur head in it but its hard to get perfect.

If you get your video post it up and I'll take a crack at it. You don't need to do the wall tihng but just try to mimic it best as possible.