Some times you can have multiple studios working on a game a produce a gem but you need to have someone at the top who is making sure everything integrates well.
AC: Brotherhood is a prime example. One team working on the Liars, one on multiplayer, one on the present day stuff and at least two producing the bulk. The end result flows beautifully and whilst not my favorite Assassins Creed game it was certainly impressive for a years work. But it only works because someone was able to corral all those studios and say "This is the game we are making!" Without that you get Aliens, a disjointed affair that doesn't really work together, good game mechanics that are in MP but not SP, and more importantly no one willing to hold up a hand and say mea culpa.
"I refuse to believe that the afterlife is run by you; the universe is not so badly designed"