How do you play multiplayer split screen on halo reach

On the main screen hit main menu, sign in 2 accounts (one can be a guest), and go into either custom games or matchmaking. Custom games is a usually local (played with only people on your xbox) and customizable gametype. Matchmaking is played with anybody on xbox live (so you must have xbox live gold to play) and gameplay is generated from the online game server and is voted on by members in the game type. Each of these will be split screen automatically if you have 2 people signed in.