PowerBook computers have always had keyboards that will not recognize more than two keys held down simultaneously; otherwise known as two-key lockout. Modifier keys, such as the option, command, or control, are not affected by two-key lockout.
The Apple Extended Keyboard, Extended Keyboard II, and Apple USB keyboards do not have the two-key lockout. If you are using an application that requires you to hold down multiple keys, you can use one of these keyboards.