Let’s Go firmly upholds the Nintendo Switch’s philosophy of “sharing the joy”. At nearly any time during your adventure, you can pass the second Joy-Con (the one you’re not using) to another person and they can play together with you at the same time.
For the second player to join in, all they have to do is shake their Joy-Con, after their Joy-Con has synchronized with the Nintendo Switch. You can generally synchronize a Joy-Con by pressing any button and waiting for the LED lights to settle. For it to work, the LED lights must display 2 lights, indicating the Joy-Con is “Player 2”.
Also, there must be a co-op play icon visible on the screen. This will appear in the bottom-right corner when exploring. Or halfway up the screen on the right during battles. Occasionally, this icon will be absent; for example, when you’re in menu screens, in the middle of dialogue or traversing water.
What Player 2 Can (and Can’t) Do
The second player can walk around the screen to follow the first player. However, that’s all they can really do when exploring. They cannot interact with wild Pokémon (to trigger battles) or NPCs. Also, the camera will only follow Player 1, so it’s possible for Player 2 to get left behind. If Player 2 gets lost and cannot return to the screen, the game will teleport them back after a while.
During wild Pokémon encounters, Player 2 can actually be of help. They’re able to use and throw Poké Balls, as well as items. But they can’t flee from the fight or access the Help menu. The main benefit of having Player 2 around is when both players throw their Poké Ball at the same time, there will be an increased chance of catching the Pokémon. This will also grant a “Sychronized Bonus” in the rewards screen.
Additionally, Player 2 can participate in trainer battles as a “support trainer”. They will control the second Pokémon in your party from the start. Once Player 1 has selected their move, Player 2 can choose their move. Player 2 can do anything Player 1 can do, except use Partner Powers. If Player 1 doesn’t use Mega Evolution, Player 2 will have that choice.