How do you turn on coop for Lego Harry Potter 5-7 3ds?

To enable co-op mode in LEGO Harry Potter: Years 5-7 on the 3DS, simply have a second player press the Start button on their Nintendo 3DS while the game is in progress. This will allow them to join as a second character. You can switch between characters at any time by using the character switch button, making it easy to collaborate during gameplay.