Lentia 2000 by Siegfried Holzbauer
This photo is part of Instant Mem 2012 - a DIARIUM-Project by s.holzbauer
( www.advancedpoetx.com/instantmem.html | flickr )
His explanation on how to make multiple exposures with a fuji instax camera:
"open the back door of the camera, put the black sheet into the gap between the upper side of the film cartridge and the camera, take as many shots you like, remove the black sheet, take the final shot and the picture will be ejected."
From my own experience it's an idea to cut a 1-2 cm strip of the dark sheet, using the whole dark sheet makes it harder to close the back door.
You might not want to do this in direct sunlight, but generally the instax construction doesn't leak light even with the back door open, so you don't have to be paranoid about getting light leaks on your film pack.