You remember a lot of photos and movies of people talking to themselves -- something like this? It's an easy setup; a person on the left side of the picture is talking or interacting with what looks like an identical twin in the right side of the picture. This is actually quite easy to do on a Mac. If you have an iSight, follow these instructions (though it's also possible using a digital camera):

1. Open Photo Booth.
2. Position the Mac so that it won't move (if it's a laptop).
3. Position yourself so that you are on the left side of the image. Try not to cross too far over the middle of the frame.
4. Snap a photo of yourself (preferably looking at your other half).
5. Position yourself on the right side of the frame.
6. Snap another image. Redo it if you're worried about overlapping your first image.
7. Open both images in Preview.
8. Using the Rectangular selection tool, select the whole image and then narrow it to only one half of the screen. Copy it. (You can also use the lasso tool for more advanced shots).
9. Switch to the other image, select inside it and paste. Move it around to line it up properly with the first.

If all went smoothly, you should see a person looking at him or herself. For a more professional look, or to get some more image interaction, try the lasso tool, and remember to save your finished product.