We've tried it in training before, hardest thing is keeping your balance as you cant see when the other person is going to dig the paddle in. For better resistance training, try it lying down and paddling away from each other - not as entertaining, but damn hard work. Angie and I kept paddling hard for 20 minutes in the one spot! Hard work but a good workout.
Great idea. Power and balance required to win. You just need a good way of determining the winner when it's not in a pool - a large floating ring of some kind?
Maybe if you wear the leagrope from the other persons board. The winner would be the one not getting dragged through the water behind the SUP board. Don't think it would work practically but would be great to watch