I saw this one day at Noosa, the 2nd guy was holding a short rope, maybe a board leash. It worked fine, they were having fun with it. I remember thinking the 1st guy would have been a little overpowered alone, but not seriously.
I've taken people body dragging (hanging off my harness handle) behind a 10, 14, 18. Even the 10 has pulled the second person surprisingly well. I'd guess once up and planing a board would provide less drag than a stomach..