Welcome and yes this has been discussed over and over, but as a Scag user I can say that if you have a 14HP on your 36" it will work fine as my 36"with the 14HP has no problems. Also you will have to remove one of the spacers as the blade bolt is not long enough to compensate for the added blade. The added blade is almost the equivalent of one spacer. Any more questioons feel free to e-mail me at oscapes@yahoo.com
P.S. If you have the 12.5 HP, then what Firebrand says is probably true. I can only speak for the 14HP. I also run the doubles on my 52" with a 17HP Kawi. No matter what if you're using the double blade set-up if the grass is really high or thick you will notice some bogging down of the engine. Also...GREASE YOUR SPINDLES OFTEN!!! as the added load seems to burn it off all the more quicker!!!