You have to get two kills within two reloads 20 times to get this achievement. This is best done in Coop vs AI on Casual difficulty on King of the Hill. Good maps for this include Forge Blitz and Hotel where the Torque Bow is found in a high place.
Wait til the enemy is standing still or are out in the open (which happens all the time on Casual AI) charge up a shot and fire. Sometimes you can shoot the floor beside covered enemies and the explosive damage will kill your target.
Note Executions count towards this achievement so long as both kills are done within two reloads (eg shoot one guy, execute the second guy before firing a second shot). Alternatively you can get all your kills by downing enemies and then executing them with the Torque Bow.