It's part of an interview featured in the current issue (95) of OXM on news stands right now. Initially the co-op feature was going to be a driving mechanic in the original Assassin's Creed, but Ubisoft abandoned the concept. "Co-op was one of those big things at the beginning that just didn't make sense in the end," Bergeron says. "For us it was really part of the single player experience, to have in-and-out co-op, and in the end we never thought it made sense in the storyline that we had for the Animus."
Of course, once the idea became more and more about Desmond and the Animus, it didn't make sense to have someone else running around in Desmond's ancestral memories. "There was no way to reconcile having multiplayer or co-op in an ancestor's memories," Bergeron explains."Your ancestor lived his life in a certain way, so assuming you had branching storylines, it creates a paradox. It didn't fit."