Start a special ops mission (split screen or coordinated over live) and choose "Stay Sharp". There are a total of 34 targets to hit -- meaning that each of you will need to hit 17. Keep track of how many you and your partner hit, either by counting out loud or alternating kills. After you finish the last room it will tally up your kills. If you both got 17 the achievement is yours.
Or, to simplify things even further:
Start a split-screen special ops, and choose "Stay Sharp".
With the first player, kill every target until you get to this room:
Leave two alive.
Then with the second player, kill every remaining target and finish the mission.