This is a favorite technique of some slide film photographers. They make two identical slides (overexposed), remove them from their mounts, flip one over the other, and remount the two slides in a single mount. This is what they would have gotten by using yesterday's daily-photo iris - http://ed-k-photo.smugmug.com/gallery/6988948_kjHp8/1/538480505_FAHHb/Original
The darker color in the reverse montage, as compared to the normal image, is an increased saturation and darkening due to combining two "slides" and is the reason that slide shooters have to overexpose by 1-2 stops - else the montage would be too dark too see.