For the squadmate mode on the Wii, the player with the second can enter by pressing . The second player is limited to fire, interact/reload and cycle weapons. To exit this mode, press to end the cooperative mode. The Wii also has the controls designed for the Wii Zapper. Check the button layout and motions you're going to use if you decide to choose the Wii Zapper as an alternative control method instead of the Wii Remote and Nunchuk.
From StrategyWiki, the video game walkthrough and strategy guide wiki