if you are buying these engines new, just get a 18-20ish HP twin cylinder. you get more good torque and less weight. you'd have enough left to get a comet torque converter thats more than capable.
you're going to spend LOTS of money on two new engines, and if they aren't running at the same speed, you will be replacing lots of belts or worse, you'll really only be pushed by one motor. I'm 300lbs, my cart weighs in at at least 400lbs by itself, with tires/motor/trans. a 17.5(500cc's) pushes me well past 40mph with 23" ATV tires.. If your wanting to go fast you really don't want 2 motors anyway, the extra weight will make it a pig in offroad slow speed turning especially if its live axle. if this is for the track, I would find out if they'll let you run it first.