Army of Two

The "Back-to-Back" action is not enabled by the player, It is part of the game so as you go playing the game these will pop up. While in "Back-to-Back" mode just kill as many enemies as you can. For the achievement you will need a total of 50 kills (does not need to be in one "Back-to-Back" action. While you are doing training in the beginning of the game you will encounter a "Back-to-Back" sequence all the kills you get in that do count, so if you want you can just keep doing training over and over.

Back-to-Back is not something that the player activates by playing, this is part of some scenes in the game, what I recommend is whenever it happens to kill as many enemies as you can, if you have beat the game and still haven't managed, then go to the Aircraft Carrier or Afghanistan mission, after parachuting you will start a Back-to-Back, kill as much as you can and repeat the mission until you unlock the achievement.