I looked at the parts book and your car does have a dual water valve.
There are a couple of flapper doors beneath the dash that are often suspected but rarely found guilty of heater problems. I have never touched one on a BMW, which is good because they are likely tough to get at.
The wheel at the vent in the dash is a simple mechanical valve. I have not seen one of those fail, and you say you are getting no heat at all on the right side, and that wheel impacts only the vent it is on.
Looks like your car uses the later style climate control computer. Although not related to this problem, you may want to bookmark post on my web page for future reference: