There was a series of videos done by a man name Phil Mikelson, who was a long time engineer for Bombardier, back in the 80's that covered all aspects of Ski-Doo snowmobiles.
One video concentrated on the twin cylinder RV motors and how the PTO cylinder had a tendacy to run a hair leaner due to the Rotary Valve rotating clockwise and cutting off the passage starting from the bottom up. The theory being that the atomized fuel did not evenly distribute in the bore but had a tendancy to hover in the lower 1/2. The rotary valve closed off this lower half first, in relation to where the piston was to the transfer ports, on the pto cylinder and last on the mag side.
Don't ask me why. I always thought the same amount of fuel and air would be pulled in and richening the pto would run the cylinder richer, but if you jetted both the same the pto ran a little leaner.
On a single carb you can't stagger jets so the cylinders aren't balanced, but run well enough to not worry. Plus, a single carb has better fuel atomization so the fuel staying in the lower 1/2 of the bore isn't as extreme and bn carbs are different from the round slides sleds use.
