Here is a Sonia screw-in double exposure filter, image splitter, double mask.

This special effect filter allows you to expose each half of one piece of film separately. One half of this attachment is completely black and blocks all light transmission while the other half is transparent. With the first exposure, one half of the image remains unexposed. Then, by rotating the attachment in its holder 180 degrees and using the multiple exposure feature on your camera, a second exposure is made on the unexposed half of the film.

These work on any size camera from half-frame to 35mm to medium format to large format.

No one knows what to call these things.  It's sort of like a filter, because you screw it into the front of your lens, but it isn't a filter -- but you can use it with any other filter.  Some call it a filter, some call it a mask, some call it a splitter or image splitter.  It has been labeled a half frame lens attachment, a double exposure filter, a special trick maker, a special effects filter, a dual image maker, a split image filter, and a double mask filter.

Take a look at the first picture.  This "filter" is half black on one side and clear on the other.   It screws into your lens and then it can be rotated with its tiny handle, just like many polarizers.  First, you take a picture with the black side covering up what you don't want exposed.  This can be on the top, bottom, side or anywhere in the scene.  Next, you need to rotate the "filter" exactly 180 degrees and expose the other side of the film.  You can take the second exposure in the same spot (the easiest approach), or you can move the camera anywhere you want to expose the second half of the picture.  The Sonia is designed to create a gentle, soft, diffused merger in the middle of the two exposures.

It's best if you have a tripod, cable release and a camera that can take normal double exposures because the film can't advance between the two exposures, and you still need to cock the shutter to make the second shot.  After the first exposure, you set your camera for a double exposure -- if your camera has that feature.  For many cameras that don't have double exposure capability, you can achieve the same effect by simply tightening up the film by rotating the film rewind knob (after the first exposure) until you feel slight resistance.  That means the film is taunt.  Then hold the rewind knob in place with your left hand.  Then press the film rewind button with your right hand and cock the shutter with the film advance lever.  Now that the shutter has been cocked you can let go of the film rewind lever and button.

For exposure, you'll need to first get an overall exposure reading of the scene from your camera without the "filter" in place.  Then set your camera for manual exposure mode, assuming it has it (this is essential), and dial in the original exposure settings.  If you happen to be using a hand-held meter you can leave the "filter" in place for both exposures.  Repeat this process for the second exposure.  If you take the second half-picture at a different time or different location chances are the two exposures will be different, but that's OK.  They will come out correctly exposed on the film.  So your first exposure might be, for example, outdoors at 1/250 at f16, and your second exposure might be indoors at 1/30 at f2.8.  That's not a problem.  Both pictures will be correctly exposed and the picture will come out fine.  People will wonder how you did it!

These "filters" are similar in use to the Cokin A 346 and P 346 double exposure filters.  These up for auction are made by Sonia, a company in India.  The available sizes are 49mm, 52mm, 55mm, and 58mm.  They all have the same size thread on the front as it has on the rear.  If you have a different size thread on your lens, all you have to do is pick the next larger "filter" and purchase a step-up ring for your lens to the larger "filter" size.  All of these filters come in a plastic resealable baggie, in the original box. 

NOTE: This filter is designed to be used with cameras that allow for multiple exposures.  If your camera lacks double exposure capability but uses film, you can try this trick:

1. Take your first exposure.

2. WITHOUT pushing the rewind button, turn the rewind crank until the film is tight.

3. Hold the rewind crank in-place with your left hand.

4. Press the rewind button with your right hand.

5. Turn the film advance/shutter cock lever.

6. Take your second exposure.

When you buy, just tell me what size you want.  If you want more than one at that point -- the same or different size -- you can purchase them at a 10% discount.