Well, it would certainly be difficult to win some fights without your teammate. You can have a party of 4, but there is a talent called lone wolf that gives you some serious buffs if you are either alone, or only have 1 other party member. Though, even then, you really do need that other party member. 

I currently have a co-op game going with a friend. We work together well. I talk to people while he picks their pockets.