You need to complete all of the activities on the map in co-op. You both need to be in the activity, so when one person starts the mission, the other needs to join him with the on-screen dialogue box. You can't check which activities you've done in co-op, just the ones you've completed period, so it's best to just start up a new game and complete everything with a friend. You do not have to complete the Hitman, Chop Shop, or Racing activities to get this.
Please see the individual activity achievements for additional help.