There is only one condenser and compressor, but two evaporators in the system. In the old days of R-12 there was a lot of concern about chlorine disassociation, collection and line corrosion from the inside resulting in failure. Not so much of an issue with FC-134a, but still a good idea to still run it occasionally.
I've never had the systems apart sufficiently to test this theory, but as there are no isolation or control valves (that I know of) in the system, I strongly suspect that freon flows thru the rear evaporator just as it does thru the front evaporator whenever the compressor is running. The only thing you are really controlling is the fan in the two separate air boxes.
If true, when you first turn the AC on freon flows thru the expansion valve front and back. If the front fan is on, warm cabin air evaporates all the liquid spray exiting the expansion valve, and the thermocouple attached to the evaporator instructs the expansion valve to remain open. So it continues to flow...
With the fan off in the back, the evaporator overchills quickly, and the expansion valve terminates freon flow. Eventually the evaporator warms, and you get a few seconds of freon flow again. Stir and repeat......
So, if theory holds true, the system in back does take care of itself in front-only operation.
Heat is a slightly different matter, as there is a real valve back there controlling antifreeze flow. Maybe run that occasionally.
