How seperate should the left and right heater controls be on my 1993 E34 - standard round dials and fan dial?

The reason to ask is that my drivers side heater dial turns with slight resistance as if its doing something, but the passenger side turns very freely.
Additionally, it seems that the drivers side dial is doing all the work - ie: it seems to make change the hot and cold air on the passenger side exactly the same as the drivers side.

Its a bit hard to tell, but turning either side hot and the other side cold doesn't seem to make much difference - whatever the drivers side is set at is what they both end up being.

Air to the feet on either side is very weak and not impressive on either side.
Air to the centre vents and windscreen is good and strong.

Any advice?
Any common problems that this forum has discussed previously?
Any more advanced testing I could do to see if there is a problem or not?