For the record, I've been unable to find *any* report *anywhere* stating that it's 2 players per cart.
Every report, including IGN, Gamespot, and more, state 2 characters per cart. Not 2 players.
Unless someone can provide a link stating that one player can drive while another can shoot, I wouldn't believe it. I wouldn't even speculate it, as it would be a really weird setup. As it stands now, 1 player controls 2 characters in 1 kart.