Not to be a self promoting *****, but check out this link:
http://www.youtube.com/watch?v=qhRw8BjooVc.
The second shot you'll see involves the same actor playing two roles, similar to the way you describe. Each in different clothes, one with a mustache.
Step 1: Determine a vertical line on the screen to use as a reference. Whatever you do, don't cross that line with any movement! All of your actor's movement and actions need to stay on that side.
Step 2: Film the actor on the right side of the line doing something. You might want a stand in whom he can react to. You might also have to time out certain reactions, depending on what the scene calls for.
Step 3. Film the actor in wardrobe #2 on the left side of the line, doing something. Again, you might want a stand in who will be removed in the next steps.
Step 4. In premiere, overlay these two video clips in your timeline so that the "actors" would be facing each other. Select the top clip, and apply the "crop" effect. This will allow you to crop off part of the video clip. Basically, you crop out half of the screen on the top video, and now that half of the screen will be visible on the bottom video. If everything works, you should have both actors side by side.
The keys to this: KEEP THE CAMERA ON A TRIPOD! And, don't cross that line, or else it will look like an actor's hand is disappearing into thin air.
You can also try this with the point matte options and keyframes, if you have a little movement in the shot.
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"Sure as I know anything, I know this - they will try again. Maybe on another world, maybe on this very ground swept clean. A year from now, ten? They'll swing back to the belief that they can make people... better. And I do not hold to that. So no more runnin'. I aim to misbehave."