Josh I have the 5.4 gasser motor and might not have those same ports but your method does give me the idea to look at my own set up. I'm running my rear heater in series and even in those below zero days both heaters work fine. I believe when the rear core blower isn't running the heat isn't really lost or reduces the front core's ability to warm the cabin.

Thanks though----might be an idea I can pursue anyway!