Michael, correct me if I'm wrong, but it sounds like you're asking how to use the front/rear pairs for music when the surround sound is not in use.
All of the following assumes that the 12-channel amp is
not being used in the surround system. If it is, get a 5- (or 6-) channel amp for just the surround system. It'll sound better and last longer (and so will the whole-house amp).
If I'm correct, the tedious way to do what you want is to figure out how to switch four speakers between two different sources. This calls for lots of wires, relays, headaches, and overloaded amps.
May I suggest another option:
Let the surround speakers stay attached to whatever is driving them while in surround use, and feed the whole-house signal into the A/V preamp/amp or receiver, using an aux. line-level input.
If the whole-house amp has no line-level feed through, you can try either Y-cords at the inputs, or speaker-to-line level adapters, such as used to connect external amps to car stereos that have no line outputs.
This way, you retain the 12 channels for their whole-house use, no speaker-level switching, and you can select whichever processing sounds best in that room, such as front/rear stereo, etc.
Larry
www.fineelectricco.com