You can search (using the search feature) these forums for "keying" and "matte". Also, search your editing software manual for the same terms. Essentially, you'll key out part of one track and overlay on top of another, allowing different parts of the higher and lower tracks to be visible. Both tracks will probably have to be shot greenscreen if you want it to appear that there is interaction, otherwise a garbage matte will do. Do a search on any of these terms (through your manual, these forums or Google) and you'll get all the gory details.