I have for sale my custom Instax 210.
Besides working like normal, it has a tripod mount, a remote shutter release and a mutiple exposure switch.
To use the multiple exposure switch:
1. Turn the camera on
2. Set the light/normal/darken
3. Set focus
4. Turn on the switch (down)
5. Take your photo
The photo will expose as normal but the motor wont eject the film. Wait, then the camera's little LCD screen will start flashing to show an error. Now you can either take another photo or flick the switch to the off position (up) and take your next photo and it will then expose and eject. The film counter will be correct too. You can take as many exposures as you want and also change the lighten/darken for the proceeding shots however you'll have to remember where it was set because the error will be showing, however the camera will still functionally change the exposure/focus if you press the buttons.
Feel free to ask any questions.