dual cd decks switched

The route I would go is set one unit up to be the "Priority" unit.

On the remote turn on wire of the Priority unit install a SPDT (or SPST) relay. 

Wire Pin 30 to switched ignition that turns the two decks on

Now, whenever you turn on the Priority unit it will energize the relay, shutting off the other unit (taking away the switched power).  It will also energize the 2 4PDT relays.  When you shut off the priority unit the relay will unlatch and switch back to the other unit.

The 4PDT relays will be connected to your speaker output wires.  The 30 terminal will be the speaker output.  Pin 87A will be the speaker outputs from the non priority unit (connected when the priority unit is off).  Pin 87 will be the speaker outputs from the priority unit (only connected when the priority unit is powered).  Do this for all 8 speaker wires and you should be set!

I can't think of an easy way off the top of my head to always switch back and forth based off turning each unit on.  I could do it easily with a small microcontroller circuit, but with relays it would be a bit more difficult.