A great way to do this liv

A great way to do this live is with a video mixer.

Video mixers support 2 or more channels, and include a chroma key effect. Chroma key one channel, and have the other channel beneath it.

Here’s an example of a standard mixer:
http://www.focusinfo.com/solutions/catalog.asp?id=10

These machine can run pretty expensive though, and the older ones only deal with analog video, so anything complex (and importing) would be a little more difficult.
Nonetheless, just having a video playing in the background and having your subject chroma keyed, a video mixer would do the job well and relatively easily.