The option to play split screen with up to four players returned in Call of Duty: Black Ops III, but only in local modes. In Call of Duty: Black Ops and Call of Duty: Modern Warfare 3, when using split screen online, only one additional player can play, allowing two players to play on the same account.

All you need to do is follow these steps: Connect a second controller or keyboard and mouse to your console. Sign in to the other player's Activision account or create a new one. Split-screen will be enabled.