[Phoenix3000]

1) Set up a new profile (or use an existing profile that hasn't played R6V1) and sign on that profile using the first controller (player 1).
2) Sign on your actual gamertag (the one you want the achievement for) using the second controller (player 2).
3) Load up the game and play split-screen Terrorist hunt missions.
4) Continue playing until you level up the player 2 character (the one with your actual gamertag) to Private First Class - Achievement Unlocked.

hoped this helped