We dual wand ALL THE TIME with our truckmounts. It simply makes more sense to dual wand on the larger, and the medium sized jobs, as time is definitely money.
One (it's electric!) of our TM's uses completely separate high-output vacuum systems, each with their own APO, and it uses the same HOT LP heater (280º capable), and the same pressure pump. This system can keep 240º at each wand (06 flow per wand) while cleaning all day long, with the heater still not dialed all the way up.
The other one uses a single over-driven #45 blower, but has separate recovery tanks, and uses the same HOT LP heater for both wands, and the same pressure pump system.
In my 24 years of carpet cleaning, I have tried many different brands of TM for dual wands. The biggest problem with them was NOT ENOUGH HEAT for 2 wands at the same time, or not enough vacuum so 1 wand had to stay on the floor.
IMO, a dual wand rig should operate without either tech being able to tell someone else is using the machine when they are. You also need the heat to stay up to where you are used to it being when single wanding, when you are dual wanding, and you shouldn't have to fuss with the wand so you or the other person running a wand has sufficient vacuum while cleaning.
I don't really see a need for separate pressures as we usually do just carpet, or carpet and upholstery (jet by-pass tool needs no dial-down of pressure for upholstery) at the same time.
If we have a lot of upholstery, and carpet, then we both tackle the upholstery first, and then go to the carpet.
On tile cleaning, we will usually schedule the tile clean as a separate job, since tile as an add-on would usually take too much time for the days schedule.
I can honestly say dual wanding has enabled us to do an enormous amount of work in a day.
BTW, we have been dual wanding for over 17 years now.