I was just looking at iMovie, and it looks like you may be out of luck. I didn't thoroughly look through the effects, but like what kkmac was saying, you would want to look for "split screen" or "picture in picture" type of effects. iMovie only has one video track, which leads me to doubt it supports these effects.
It sounds like you want to do a lot with your NLE, and with a better one, it may open up much more creativity. The next step up from iMovie would be Final Cut Express HD for $299. This NLE is very, very, very similiar to Final Cut Pro, and supports the effect you described and much more, including: green screening (chroma key), muzzle flashes, explosions, and other sfx.
The downside to this, however, is that FCE is fairly complex, but is worth to know.
As with any typical NLE, the camera must be stable, and try to keep the characters from crossing each other.
Goodluck!
PS, if you have a PC, I know pinnacle studio 10 can do the same effect, for a cheaper price. (Just be warned that Pinnacle crashes A LOT!)