The dual A/C and the refrigeration cycle with rear cooler have evaporators and expansion valves for front and rear. These make the refrigerant circulate using a compressor.
To control two refrigerant circuits, solenoid valves are set up.
When the front A/C switch is turned on, current passes through the front solenoid valve and it turns on (opens), while current does not pass through the rear solenoid valve, so the valve turns off (closes). Therefore, the refrigerant flows only in the front side circuit.
When the rear A/C switch is turned on, current passes through both the front side and the rear side and both of the valves turn on (open), so refrigerant passes into the refrigerant circuits on the front side and rear side.
In some models, current passes through only the rear solenoid valve when only the rear A/C switch is turned on.
Automotive Air Condition System Dual A/C (Rear Cooler) Control