i am not 100% certain of all of the little actuators and controls on the hvac system for the 7, other than my own e32. they only get more complex, though, as they progress.
anyway, in general terms, dual control systems, be they on bmw, mb, or domestics, have a series of blend doors and actuators (control devices), temp sensors and settings (inputs). when you get a situation where one side is delivering the requested temps and the other is not, my first check would be to verify the full operation of ranges on each side.
you could have a temp sensor that is reading incorrectly, blend door motor that is worn, or even an obstruction that is restricting the proper operation of the blend doors. or something else altogether.
ac systems are expensive to fix, and have always been, but they tend to be one thing that people have become accustomed to and will refuse to do without.
i am sorry that i am unable to give you a 'simple' fix, but i believe that your guy checking the vehicle needed to have a better understanding of the system before scratching his head and stating the refrigerant level appears to be sufficient. not that it was a wrong thing to do, but it was a 'zig' when he should have 'zagged'.
i would check with the fellow festers to see who in your area is recommended for ac repairs and go see them.