Originally Posted by The Pants Party
Friends with Benefits 50
Complete all acts in co-op on any difficulty (Marcus or Dom)
I would suggest coupling this with your Insane difficulty run as this will make it much easier. If for some reason you have to do this out of order, that will not affect the achievement. You do not have to be the second player like in the first game, both players will get these achievements.
I just wanted to add-on to this, if you host a public co-op game and no one joins, you DO NOT get credit torwards any of the co-op achievements.
I learned this the hard way also when a person joined my game and left before I finished some chapters.