You can get this achievement at the end of the mission Hometown. You will need to grab the handgun from the table by using the mobile phone to distract the soldiers. After I got the weapon I went and hid underneath the truck, I then came out after a while and they were both lined up. Overall this achievement is not really that difficult, it’s just a matter of patience and timing. You will also get a checkpoint just after you pickup the handgun so that you can try again if need be.
If you are still struggling, feel free to use this as a reference.Call of Duty: Modern Warfare Two Birds Achievement Guide