Oops. You guys, I was wondering this too... There is a seperate control for that middle vent. The dual a/c for the e36 a/c seems to really be a triple. If you look below that center vent, in addition to the i/o wheel, there's a wheel that has some blue dots and red dots. That controls what comes out of that middle vent. If you turn it... uh, right (I think), so that the red dots are showing, hot air will come out. If you turn it left (I think) so that the blue dots are showing, cold air will come out. Turn it to the middle so the line is visible to get a mix.
Yeah, I know, it's weird that this is independent of the two other controls (left for driver, right for passenger), but it's also pretty nifty. More like triple zone a/c.
1998 BMW e36 M3 Sedan
Crap, I think I may be a yuppie...
2003 MINI Cooper