Don't use a keyboard hack for 2 players. Just my opinion that its not worth the time and effort. Save yourself the headaches and buy a $27 keywiz encoder. If that is to steep for your budget do a gamepad hack. If you go with 2 USB gamepads though its likely to cost as much as the much better keywiz.
MS Sidewinders gameport version are very nice to hack. They can be daisychained(up to 4). They can be found usually for under $5, they have a common ground, 14 inputs per pad, nice marked solder pads, under win98 the driver can be mapped to keyboard input.
Go with the keywiz....