I think you would get pretty decent power on a go kart with two pocket bike engines, one on each rear wheel. Don't worry about a jack shaft or live axle. Just chain one to each wheel. On the side where the sprocket is on the "incorrect" side of the engine, just mount the engine in front of the tire to make the sprocket line up. Don't worry about the kart pulling to one side, either. One wheel drive go karts don't have problems, so there's no reason to believe that independent dual engines would have trouble. They've been building airplanes like that for a long time, and just a tiny bit of rudder trim evens out any difference in thrust. Your front tires will keep the kart straight and you'll never know the difference.
If you want to make it even better, get a CVT. Check out this ebay link
http://cgi.ebay.com/ebaymotors/2-ST...088754QQptZMotorsQ5fATVQ5fPartsQ5fAccessories
I bought several of these. With a 4:1 final ratio and standard 10 inch pocket bike tires, my mini bike reached a top speed around 25 on pavement, with good acceleration. I've been thinking that two of them on a go kart would be a load of fun. And you can't beat the $20 price.