Very important- If you have not unlocked the Dark Agent difficulty, you will not get this achievement, regardless of whether you actually complete the game in co-op on Dark Agent. Therefore, both you and your partner need to have Dark Agent difficulty unlocked to be awarded the achievements.
Refer to "Completed Single Dark Agent" for more information on the Dark Agent difficulty and mission guides. This can be done locally or online.
When playing co-op, it is possible to revive your partner if they go down. To do this, stand by their body then press or on the D-Pad to begin the revival process. You must stand by their body for 10-15 seconds, otherwise the revival process will need to be started again. There is a rather large drawback to reviving your partner: Half of your health will be taken away.
For the majority of levels, only one player needs to reach the mission exit. The exceptions to this are:
Mission 00- Datacore
Mission 02- Subway
Mission 03- Rooftops
Mission 05- Laboratory
The missions are not much different in co-op, there are no extra enemies or objectives. There are however a lot of doors that require both players to be present to unlock them (two buttons). There are also elevators which only allow one person to ride in them. Usually there is another button at the top/bottom to send it back for the other player.
Complete all 14 missions in co-op, on Dark Agent, to unlock this achievement.