I guess I posted a simple but never-ending question.

Yesterday, while driving around town, I put the passenger side to 90 degrees and the driver side to 70 and definitely felt warm air coming out of the rear passenger side vent. The rear driver side vent seemed to be much cooler air, but not as cool as the vents in the front. I'll have to do more testing and swap the temps to see what happens. Right now I'm inclined to believe they are separate, dividing the car in half in "dual" mode.