Dolphin, the GameCube and Wii emulator - Forums

Maybe you could set it to 3 player, and have the third player's part of the screen off the real-life screen. It may mess up scoring and stuff, but as long as it's only one player getting a DNF, the game should be able to end each race properly.

or maybe make custom resolution for your screen

http://forums.guru3d.com/showthread.php?t=346325