From Brian Quinlan/ Joe van Tunen:
Keyboard Combiner fixes a problem with the Macintosh Operating System's handling of input from Apple Desktop Bus (ADB) devices, especially keyboards. Normally, your computer can recognize the button presses from only one keyboard or joystick at a time because pressing a button on one device causes it to replace the buttons on other ADB devices. Most of the time this is not a significant limitation but during game play, it can be very annoying. Imagine a two player game of Ambrosia's Mars Rising being played on a computer with an Apple Adjustable Keyboard (which is actually two different ADB keyboards). If one player is holding down a key on the keyboard part to move left and the other player presses a key on the keypad part to fire, the first player stops moving. A similar problem occurs when playing games with both a keyboard and a joystick. Keyboard Combiner allows your computer to recognize keyboard and button presses from all the devices connected to your ADB at the same time.