Two modem with null connection either will not work, or will be slow, and painful to set the games.
Much faster, smoother, and ?Yeah Cooler?, is to get two NIC (Network Cards, they go for $5-$10 a piece). You add to it CAT5 Crossover cable (few $$$), and for a sum of $20 or so you have a Cool small Local Area Network (LAN).
Great for games.