Instructions
Connect one end of your Crossover cable to the Ethernet port on the back of your first Xbox.
Connect the other end of the cable to the Ethernet port on the back of your second Xbox.
Turn both Xbox consoles on. Insert your "Halo" game discs into each of the Xbox console's disc slots.
Select the "Multi-player" mode in one of your "Halo" game's main menu, and choose to host a game.
Select the "Multi-player" mode in your other "Halo" game's main menu, and choose to join a game. You will now be able to play a multi-player game between the connected Xbox consoles.