To unlock this achievement, you and your partner need to kill 2 targets at the same time, while the targets are unaware. To unlock this with the AI would be very difficult, but it is possible. It is recommended that you do this with a human partner, either in split-screen or online.
A good place to unlock this is during the mission 'Cause and Effect'. The first enemies you will come across, there will only be 2 of them, and they are looking away from your position. You need to line up a headshot on both targets and shoot at the same time, so that they both die instantly and have no chance to raise an alarm. Once they are both dead, the achievement will unlock.