Yes it is normal.
You are right that hot always flow through heater in most Ford cars. Including 'Dual Heater'
There is usually no outside air into the rear unit, it is recirculate- only
Many engines count on the heater flow for the coolant bypass necessary during warmup. It's important that you dont shut this off completely. So you have to make sure there is a circulation path for the coolant before the thermostat opens, in fact the thermostat works better in this scenario.
THAT said, you can go to HD or Lowes and get a valve like the one you referred to and install it in the line to the REAR heater assy, as long as the front continues to recirc coolant. It really wont matter which line you put it in, as long as flow is stopped. Look for the valves that come manufactured with the barb nipples.
If you stop flow to BOTH cores, you have to make sure there is a continuous flow from the top of engine back through waterpump. If not existing, and you want to shut off ALL heater flow, then you need to put in tees on the engine side of valved heater lines and allow small but reliable flow path, suggest 1/4 inch line minimum. Like I said, the latter is not necessary if you only shut off flow to the back.