That has got to be one of the neatest...and most unexpected...things I've seen in a while. Who was it intended for? Kids? Tiny adults? Circus freaks? It doesn't look as though the front and rear cranks are, or ever have been, synchronized. I guess the rear rider does all the pedaling?
I can't help with the identification of your bike, but a quick Google search revealed yet another tiny tandem for sale at this very moment--a 16" Schwinn with a serial that decodes to Dec 1980. (And no, it isn't a Mini-Twinn.) There's actually a demand for these things?!