Hi Arcdude,
I have run several boats on snow from a big swamp buggy with 4 air rudders and single rudders. I don't think it matters if there is one two or more as long as it is in the prop wash, and I do prefer them hanging beyond the transom as that is kind of your pivot point and the further back the more leverage the rudder will have - start big and trim it down to the size that gives the control you need. I use a nylon ball joint to attach my servo arm that has a break away ability - make a sturdy rudder attachment and a light weak rudder - you will likely break a few in a blow over.
Also I have had snow that is greasy fast and super hard to control and wet snow that sticks so bad you can barely move.
Best, is about 6" of FRESH light powder on a frozen lake - I have seen some pretty incredible speeds and nothing like driving by your ankles at 40 mph for a sensation of speed -Another bonus is my nitro motors seem to run really strong in the cold.