As far as my H/U is concerned, I'm not to worried about adding a second camera to mine but those links are helpful for other endeavors.
I understand what you are trying to obtain and I have mentally drawn some ideas. Just so I understand, here are some questions that will give a better idea of what your thinking:
Which input do you want to use(Rear camera or Aux)? The Rear camera input will allow music/whatever to continue playing with the camera engaged.
I know you are powering the cameras with the 16-pin +6V. My guess is that you want 3 positions with off being the spot where the rear camera works normally. Correct?
Have you spliced into the reverse wire yet(on the 28-pin harness)? you will need to trick the H/U into thinking that the car is in reverse. I suggest you do this with a splice harness so you don't butcher the factory wiring. If you don't have one, you can find them on ebay or on
www.autoharnesshouse.com.
I'm no guru, but I hope that I can be some help. An Electrical Engineering degree has gotta be worth something and I've done a good bit tinkering.