Would both left and right air blend doors fail simultaneously? How is it set up?

[90sbuickrescue]

I know a lot more about motors, suspension, etc etc than I do about the stuff under the dash. 99 regal ls. If any of you guys have any thoughts on this, let me know. Throughout the summer, the system was stuck putting out heat, and couldn't control which vents air came out of, just fan speed. So it was just climate control off, windows down. Then it was parked for a few months, and when I started it up, of course since it's winter now, no heat. At least it's consistently annoying. Why can't it break where the air is cold in the summer and hot in the winter? So today I swapped in a control unit from another regal, I don't know the year or even the condition of the unit. Now I can control which vents the air comes out of, the fan speed, read the display, put it in auto mode, etc. But still not the temp. Stuck on cold. The system has the dual zone thing, which I assume means it has 2 heat blend doors and actuators, one for each side. It just seems weird to me that neither one is moving. I'd figure one would move and the other would be stuck, but not both. I'd guess the chance that both climate control units are broken in a way that stops them from being able to control both heat blend actuators to be somewhat improbable, but I don't know. Also, if both heat blend actuators were bad, why wouldn't they stay where they were, putting out hot air?

Anyway, if you guys have any info on this, it would be appreciated.