Ok... I must've been confuzzed on the beam splitter thing.. I seem to recall a discussion about beam splitters inside CRT TV's, but, regardless..
That kind of calculation should be quite easy to do, and although I can't currently tell you how.. I guarantee we could figure it out.
In fact, I believe that some designs used a "dummy" glass in the optical path to compensate for discrepancies of this sort.
Imagine a one-shot 35mm or 120 camera! You could have 3 rolls of film in 1 camera. That would be awesome...