I have the same problem. Split screen would be a lot more fun if one person could use joysticks instead of having two people crammed onto one keyboard. Hopefully someone will figure out a workaround for this.
I had an idea.. and it worked!!! I set one of my joysticks to use some of the keyboards keys as the buttons on the joystick using the program called Xpadder... i went into the game.. used one joystick as first player and used the second joystick(The one using Xpadder) as the second player.