Polaroid film is a very convenient way to make photographs but it is by no means the least expensive way . If expense is of concern to you, but you are willing to sacrifice picture area or size, it is possible to stretch your photographic dollar by making one piece of film do the job of two thus cutting your expenses in half.
In order to accomplish this, the procedure varies slightly depending on whether you use sheet film or pack-type film. Since for most of my shooting I use a Colorpack camera and Type 668 color film, and a 4x5 inch film pack adapter for my view camera, I'll describe the procedure applicable for pack-film first.
Briefly, to make two 1/2 frame exposures on the same sheet of film you need to mask the image gate of each pack before inserting it into the camera with a piece of black, opaque material. The protective cover sheet that you pull out of the pack after loading the film can be used although I prefer stiff black cardboard instead.
The mask is installed so that if fits snugly into the image gate of the pack and is held in place with black photographic tape. It needs to cover the side of the pack which is nearest the edge where the white tabs are. The mask which I make is about 3 1/2 by 2 inches in size for the 669 type film and 4 by 2 1/2 inches for the 4 by 5 inch, type ??? film packs.
Since you've covered up 1/2 the image gate you will only expose one half of the film when you make the first exposure. In order to expose the second half of the film you need to move the exposed half out of the way and bring the half that was protected by the mask into position behind the unprotected area of the pack. To do this you remove the white tab only halfway. This advances only one half of the film from the pack, leaving the unexposed second half in position for the second exposure. After the second exposure pull the white tab out completely and process the film normally. You'll find two images side by side on the one sheet of film.
In order to ensure that you pull the right amount of film out of the pack after the first exposure I'd recommend that you install a guide, attached to the end of the camera next to the tabs that will allow you to consistently pull the right amount of film out of the camera. On my guide tab used with the 668 type film the mark is 2 and 1/2 inches from the end of the camera body.
This procedure lends itself to making passport size pictures (some passport offices will not accept coated Polaroid materials but coaterless may be ok. At one time only color Polaroid film was acceptable by US Passport Offices but the situation may be different now) and also could conceivably be used to cut down on the cost of making bracketed exposure sets when using Polaroid film to determine proper exposure. If you make pictures of family and friends you'll be able to give away twice as many pictures. Just keep in mind that since the area that you have to work with is smaller, your pictures will be better if you concentrate on filling the available view with as much relevant material as possible. That is, crop out superfluous subject matter mercilessly.
It is also possible to install the mask into the camera body rather than placing a mask into each pack. This is simply a matter of personal choice. The procedure used remains the same.
In order to use this system it helps if you mask your viewfinder down with a mask that eliminates that part of the view which is now covered up by the mask which you have installed in the pack. The side that you need to cover up will vary with the specific camera that you use but it should not be too difficult to find the half which needs to be eliminated from view.
When you photograph with the mask in place you'll find that the camera will need to be pointed slightly to the side in order to take a picture of something directly in front of you but you'll quickly become used to this. A side benefit is that your pictures may have a more "candid" look to them because people will think that you are photographing something else and relax.
If you use a pack system that uses a dark slide to protect the unused sheets of film as with certain adapters for view cameras, etc. then you can also generate two exposures by making a special dark slide that has an aperture cut into it corresponding to an area approximately equal to a half frame located toward the trailing edge of the slide. This dark slide is then inserted fully into it's slot for the first exposure and removed half way for the second exposure. A regular dark slide is obviously used to install or remove the pack from the camera.
Making two exposures onto sheet film which is used individually by way of the Polaroid sheet film adapter with 4x5 view cameras involves a variation on the above theme and is accomplished by simply covering 1/2 the image gate of the sheet film adapter for the first shot, making the first shot, replacing the protective envelope, removing the adapter from the camera, moving the mask to the other side of the holder, removing the envelope and then making the second exposure on the other half of the film.
The advantage of the pack type method is that both photographs will have the same "look" since, if they are of the same subject, they are taken from the same camera position while with the sheet film method the camera angle has to be changed to place the image on the other side of the holder. Rather than dwell on the disadvantages of these methods of multiple exposure you should realize that for special situations your film costs have been reduced by 50%...a good deal by almost any measure!