Buy the sniper rifle from the first gun shop you come across.
Equip the sniper rifle.
go around the corner from the gun shop to find a crime scene where police are investigating.
Line the two heads of the police investigating the scene as these have the least amount of health compared to the police guarding the scene.
Hack the lights to cause a distraction (if needed) to move the police into a better spot to maker the shot.
Make a save file that you can go back to after completing the achievement or if you miss the shot and need to try again.
Hold RT to charge your shot and fire to kill two of the police in the same shot.
Video guide for reference of locations and to visually see what I'm talking about.