Things You'll Need
Instructions
Turn on your Xbox 360 console and both controllers. Log each controller into a different profile by pressing the Xbox Guide button, selecting "Sign in," and then choosing the desired profile.
Insert the Left 4 Dead game disc and wait for the game to load. When prompted, press the "Start" button on your primary controller and select your storage device to access the main menu.
Press the "Back" button to enable split-screen mode (if it doesn't show as being enabled already), and then press the "Start" and "A" buttons on the secondary controller when prompted to sign both accounts in to Left 4 Dead.
Select your game mode (Campaign, Versus, Survival, or Offline Co-op) and begin playing Left 4 Dead in two player split-screen mode.