For anyone interested in geeky technical stuff here is a bizarre 16mm system.
Designed by aviation engineer Andrew Harper in the 1930s he started out by wanting to fit a 35mm sound system onto 16mm film to give it more fidelity, but then discovered he could actually fit two films on the same reel. Anyway, WW2 got in the way and it was all forgotten about.
The following clip contains two trailers which are both on the same reel BUT not one after the other these are one after in 'frame'. When the first trailer finishes you do not rewind the film you just play it again and it shows the other trailer.
Freeze the first frame of this Youtube video and you will see the Navy musical playing but in the frames above upside down is the trailer for The Lisbon Story.
Hmm... doesn't this lose a lot of resolution on the image? Not because of the extra audio track, but just to get two images in one frame? Ironically, it would have worked fine with extreme widescreen, I think?