I've done a couple of 'twinning' shots in my time, and yes, barring motion control setups you definitely need a locked off camera, though you could fake movement in post (though you'll lose video resolution of course).

For any shot where I needed one 'version' to go in front of the other, I used Ulead VideoPaint to literally draw out a blue/green solid block around each person each time, so I ended up with a completely blue background with just the twin moving around it. Comping that onto a black background plate looks surprisingly good, though make sure your person's shadows are included in the shot.

As for a 'double' moving behind the image, a simple method I used for 'interaction' for another shot I needed to show two people skipping down a path holding hands which was much less time consuming basically involved using a split screen overlay shot first, (look below, you can see the effect it had on the left guy's arm. Then I literally drew in a blue blob around the right guy's arm, so I knew it'd fit in properly, and comped that over the top of the original left guy's plate. That left a perfect looking shot :) Actually, the shot on the left is after I'd done the same thing with the shadows, you can see they look pretty damn good, even though it was a simple chop down the center(ish) of the screen.

http://www.jh-m.demon.co.uk/twinning.jpg

Nice editing on the close-ups. Looks a bit red to me though...did you white balance or try color correcting in post?

Fraid so... as I mentioned I screwed up the lighting, so you really couldn't see much at all. I had to pump up the brightness, and the white balance wasn't set correctly. Hey, it was the first day I had the camera and I was trying to be clever ;)

Still, got to learn somehow, haven't we?