I remember awhile ago there was an error on Nintendo's website listing the game as multiplayer, but it was confirmed as a singleplayer game.
EDIT: Yeah http://www.nintendolife.com/news/2011/11/luigis_mansion_2_is_not_multiplayer_after_all November, 2011
EDIT: Yeah http://www.nintendolife.com/news/2011/11/luigis_mansion_2_is_not_multiplayer_after_all November, 2011