Things You'll Need
Instructions
Play the game until you reach the third level in the seventh world, "The Great Magician's Palace." Choose to play this level with a second player, either online or connected to your console.
Play the level until you find a "2x Players" sign. Solve the puzzle by jumping into the hole in the wall. Have the second player press the buttons. This will drop large blocks and allow you to get to the top of the area. Activate the checkpoint once you get there.
Solve the second puzzle by timing your movements with the other player. When you run across the lower platform, blocks will appear to allow the second player to run across the platform. Once you both make it through, you can grab the "Grey Trainers" and add them to your collection.