The traditional way to do this, as long as each "copy" of the talent keeps to herself, is to lock down the camera, shoot each take -- say, one girl stays on the left half of the screen, the other girl stays on the right half) and then set up a vertical wipe transition that's locked at the half-way point so the wipe point is right down the middle.
Edit: Cropping works too.
You wouldn't be able to have the talent interact with herself, though (e.g. coming out of the same body). You could accomplish this with some creative editing that starts off that whole process though. Say the talent is sitting in a chair and then the clone rises up out of the chair while the original is still sitting down. You'd shoot both takes starting with her sitting in the chair, then she gets up and moves to the other half of the screen. Use a long dissolve to get the effect started and then switch to the wipe/crop to keep both copies on the screen.
Or you could use a body double and creative cuts to establish the "split" and go from there.
I think that makes sense... 🙂