HI I was always told that looking at the rear of the boat the left hand prop should rotate anti-clockwise, and the right hand prop should rotate clockwise when the boat is going forward. This will then draw water from the boats centre and cause it to pull itself down into the water keeping the props as low as possible in the water. if the props rotate the other way there is a tendancy for the boat to lift and become unstable.
I expect other will have there own views on this. Hope that what I have said makes sense. Graham