Each player will need their own set of controllers.
For the simplest way to sync to joy-cons, first remove a pair of joycons from the system. This will bring up the controller sync screen.
Without reattaching the joycons to the console, press a directional button on one joycon and any of the ABXY buttons on the other.
You should feel each joycon rumble and appear on the screen with a green player one light on.
Next attach another pair of joycons to the console. They should now appear on the screen.
Remove these joycons and they should automatically become player 2.