Just connect a second controller, and press the Jump button on it during a level. A second player will be able to play as the Bootworm from the Bootworm Racing minigame.

This can be especially useful for the tougher trophies, like "Low-Level Run" or "Remix Mode complete".

The Bootworm is invincible, so although it only deals a little damage at a time, it can help give you a slight edge in boss battles and tough levels.