Luigi's Mansion: Dark Moon Has Some Kind Of Local Multiplayer
According to Nintendo's official Luigi's Mansion: Dark Moon website, the game has some kind of multiplayer.
The listing on the right rail of the site still lists the game as a single-player game, but the description says, "With so many new places to explore, all new ways to capture ghosts, and a brand new local multiplayer mode, Luigi's Mansion: Dark Moon is another star turn for Mario's heroic brother—and it's only on Nintendo 3DS. "
Could it be something similar to the Luigi's Mansion game featured in Nintendo Land for the Wii U? The description calls it "brand new" so that seems unlikely.