I worked on this game twice, once back in 2008, and again right now. Previously, I did Agent Solo - Perfect Agent Solo - Dark Agent Solo....
I found a coop partner for DA, but when I hosted a lobby, only Perfect Agent was available to me for some reason. He had DA unlocked in coop, I joined his game and completed 1 DA level, and the achievement didn't unlock.
Sheltons method is best and required minimum playthroughs, Agent Solo - Perfect Agent Coop - DA Coop.
I think the way the game works is in order to get credit for DA Coop, you both need to run through coop PA, besides this method unlocks solo DA anyways..
Arcade Crazy - 26 - Over Achiever - 54 - XB1 Points - 573
Enthusiast/Fanatic - 5
Franchise Clubs - 9x Tier 1, 6x Tier 2, 1x Tier 3, 1x Tier 4
Staff Clubs - 12