All the movies are faking it and your life is a lie...
But seriously, most movies use this technique.
Chroma key compositing, or chroma keying, is a special effects / post-production technique for compositing two images or video streams together based on colour hues.
Basically, you are taking two images and superimposing one over the other. The green colour is the 'key' it is the background and will be replaced with a new background using software.
First, do this...
Then check out these videos
Conceptualize your scene; what actions are taking place in the background scene, and what actions are taking place in the foreground scene?
Record your background scene (use an ipad), this scene will NOT have the chroma key in it.
Record your foreground scene(use an ipad), this scene will have the chroma key in it.
Use Adobe Premier Elements to superimpose one scene on top of another.
Export the scene in a playable format (.mov, .mp4)