You could do that, or use a flash to fix one position or the other. If you are after movement, then I would use the flash at the last position. Not sure how you would trigger it, though might have to do that with a remote and your big toe. you might even trick your cam by using rear curtain flash. Not sure, have not tired that out. The only thing I have tried with long exposure is the stuff Greg and I did one night; we were trying to do some light painting, and worked with a flash to fix some positions. However, this works fine if there are two people doing the shoot. With one it's kinda hard to do, unless you have a timer or something.

If you can set your camera to do several exposures sequential, this may work fine, and then use Iggy's suggestion in overlaying the images.