Assassin's Creed 3 briefly counted "Online co-op 2 - 4" among the game's various features, as well as "Online multiplayer 2 - 8." While the online multiplayer classification remains, the co-op listing has since been removed from both the European and North American Xbox Live Marketplace.
Situations like this are tricky, because the Xbox Live Marketplace has been known to list some patently false information. At times, however, it can be right on the money, so we're bringing it your attention. While multiplayer is nothing new at this point for Assassin's Creed, campaign co-op would be a major addition to the franchise.