I have a three-way manifold running off of one regulator, and a single QD running from a second regulator. I have tried altering carbonation levels to suit various styles and generally speaking I find it is not necessary. 2.5 volumes of CO2 suits most beers just fine. Maybe if you are trying to serve something "cask style" with very little carbonation on one tap, and a highly carbonated Belgian blonde or something like that on another tap, it's worth it.
I mainly use the second regulator for force-carbing & "setting" the seals on a fresh keg (purging the headspace and then pressurizing to 35 PSI before removing the QD and conditioning.) Every once in a great while I'll quick-carb a keg, but I don't like doing that unless it's absolutely necessary (i.e. I have a pale ale that's peaking in hop flavor & aroma, and I forgot to carb it until 2 days before I'm having some friends over.)
Edit - Another thing to keep in mind is that you don't need to carb and serve at the same pressure. In other words, you can carb a Belgian or a wheat beer up to 4+ volumes, and then drop down to 14 PSI to serve and the carbonation won't go away. And with a 1/2 empty keg, you can shut off the gas and that 2.5 gallons of pressurized CO2 sitting in there will dispense many pints before it starts to slow down. So there's no reason why you can't switch off the incoming gas to your lower-carb'ed beers that are on tap while you crank up the gas for the one you are trying to bring up to the fizzy end of the scale.