Another easy place to get this is prior to the first Nemesis T-Type boss fight. You will ascend stairs shortly before you make the approach to the construction high-rise. In a small open floor between two sets of stairs you will see 3 zombies: 2 standing and 1 on the floor eating a corpse. The two on the right (eating zombie and construction vest zombie) will be easy to bundle up for one shot. Aim high and go for a headshot with the M3 Shotgun at point blank range. If aligned correctly you will destroy both with one shell.
Here is a video I recorded of me getting the achievement.
Sorry, this Game Clip has been removed