If this is an actuator problem which most of the time this is, you would only need to buy one. The drivers side temp actuator would be the problem. There is a calibration I would recommend trying. I have seen this cure many problems just like this one. Here is how this is done. Feel free to contact me with any questions!
This actuator is located underneath the bottom side of the dash. You may need to remove the underdash sound insulator to access this point.
Its actually about in the middle. Here is a view from the bottom side of the heater box.