PICO-8's controller support is based on the SDL library. A game can have up to 8 simultaneous players. Players 1 and 2 can use game controllers or corresponding keys on the keyboard.
This Pico Park: Classic Edition can only be played locally in co-op on one device. You can use the keyboard or controllers to play with your friends. To start the game, press Enter, choose how many players, and press space to confirm. You have to move your character to the door at the center of the screen.