It will not be possible. Black Ops 2 campaign is open world, in order for a co-op campaign type feature in Black Ops 2 to be released it would have to be like spec ops, which has nothing to do with the storyline. It would be really dumb to make an open-world game co-op online, but maybe splitscreen it would be possible.
Online would be very difficult, because the choices you make in the campaign effect the storyline, so let's say your squad-mate makes a very bad choice, that you would not had made, and it effects the storyline huge. Would you want that?