I've been using 2 active cabs on a Rock N Rolla cart (the smallest one with solid wheels) and my rack on top. FYI, the two cabs fit on the cart like is was made for it. I've actually had a number of people ask if the cart came with the speakers. The dispersion is excellent in that position. If you were on an extremely shallow stage it might be an issue, but I'm 6'6" and even just a step away from the cabs and I'm in the dispersion cone. I've been leaving the cabs on the cart for gigs for faster setup and breakdown. I can wheel in and be set up and sound checked in 5 mins. That's from opening the back of my truck to being ready to play. It's just as fast in reverse.
The switch on the back doesn't affect the dispersion. It compensates for the accentuated frequencies you get based on how they are physically placed. The 90degree conical dispersion (45degrees from center in all directions) remains the same. For example, the backline setting compensates for the hyped low end you get when your cab is low and projecting parallel to the floor.