Edit: Zombies has changed, principal for achievement remains.
An easy achievement, requiring a double kill with the shotgun. First off, this does NOT have to be on a noble map pack map. You simply have to OWN the DLC to get this achievement.
For easy shotgun kills, I suggest the Living Dead playlist. In a 12-player, 3-round game, you have a about a 75% chance of being human each round, you have very good odds of getting put as a human. If you do, try to hang onto your life at least until a couple other humans are infected, this way there are more zombies for you to get a double kill off. Zombies will die from quite the distance with the shotgun so try not to let them get too close.
The Alpha Zombies gametype seems to be a fair bit easier to get doublekills in because the zombies only have sprint to run at you. In the other gametype, Infection, the zombies have evade, which can prove very troublesome when trying to get close-range kills.
goodluck, and enjoy your simple achievement!