You can do the "twin" technique. These was used extensively in the 60s television "The Patty Duke Show" where she played identical cousins. Lock the camera down, mask off one half of the image/lens (say the right side). Shoot the scene normally with the actor on the left. Rewind the film. Mask off the other half. Shoot the scene again with the actor on right. If done properly the seam line should be barely noticeable. Cokin has a filter for this.
I could never get it to register properly using my Minolta X-700 film camera. In order to do multiple exposures without rewinding the film I had to 1) tension the film with the rewind knob 2) keep the tension on the film by holding the rewind know while pressing the rewind release and operating the advance lever. No matter what I did there would always be a little movement of the film, throwing the alignment off.
This should be a snap now with my DSLR. I just haven't gotten around to trying. I finally found the filter that I misplaced somewhere.
Cokin Double Exposure Filter Series A A346
Yes, you can do this in post processing but where is the fun in that?