Check the settings for your game to see if it allows for it. player 1 uses WASD, player 2 uses arrows), but you can only get as many separate keys as the game allows (for Overcooked, only 2 people can use the keyboard). Some games (such as Overcooked) allow the use of different keys for each player (e.g. This is because Windows and the game has no way to tell which keyboard and mouse belongs to who. Unlike controllers, all keyboards and mouses are seen as one. Multiple players are using keyboard and mouse Everyone must be on a unique slot for them to be seen separately by the game. If you're using an emulator, you also need to make sure you haven't mapped everyone's controllers to the first slot in the emulator settings. Search online if you're unsure about whether your game has this, and how to enable it. Keep in mind that your game needs to actually have a split-screen mode implemented.
In A Hat in Time, you need to enable co-op in the pause menu. For example, in Enter the Gungeon, you need to talk to the purple cultist in the hub to enable co-op. Some games will require you to do additional steps in order to split the controls. If co-op controls aren't working and everyone is controlling player 1 in the game, check each of the reasons below.
