I'm not going to say making such a thing is impossible, but it well beyond my skills. I would think you would be able to buy something, though; basically, it would involve (on the camera end) taking the composite output, placing it on a specific frequency, then on the other end having a multi-channel tuner to "de-mux" the frequencies back to separate composite outputs. Essentially, more or less, what a television station thru a cable line to your television does (with multiple tuners, of course). I am not sure what they would be called (except perhaps a "video signal multiplexer/de-multiplexer", of course) - they likely won't be inexpensive, though; here's an example:
**broken link removed**
The strange thing is, if you look up "video multiplexer", you get devices (which aren't cheap, either) that take multiple signals, then turn it into a single signal for display of multiple views on a single monitor (like you'd see with a security camera system) - this isn't the same thing as like what that "Gemini" product above is doing (which is what I believe you are looking for?)...