Ok, I have ran doubles for over two years now on a 36 Bobcat with a 15 hp Kaw. I liked the results. This year I notice a fine line of grass being left between the blades. Tried everything to correct, different blades, kept deck clean etc. I pulled off the doubles and the problem was fixed, no fine line of grass between the blades. I don't know if the doubles were slowing down the tip speed or what, I would think a 15hp was plenty of power. I also noticed a lot less stress on the engine, it was really purring nicely.