To play Rocket League split-display on Steam, you will  Rocket League Items need controllers. Keyboard/mouse is not presently supported in relation to split-screen play. Any DualShock 4, Xbox One, or Steam controller will paintings best.

With two controllers connected, release Rocket League. At the main menu, Player 2 have to press the Options/Menu/Start button on their controller. After that, Player 2 will join the party, and you could play Rocket League cut up-display offline or on line.

As is the case with Steam, the Epic Games Launcher does no longer support keyboard/mouse and controller pairing for Rocket League split-screen. So, you will want two controllers to play split-display screen on  Buy Rocket League Items  Epic Games. Epic Games helps DualShock 4, Xbox 360, and Xbox One controllers.