There are tons of opportunities to get this. Many times you will see two enemies standing next to one another talking, or just walking down the street together. Line yourself up so you hit them both in the chest. If you can nail a double headshot, you've made it much easier to kill both of them.
A simple place to get this is during the very first level. After you crawl and kill the enemy, you will take a right and see 3 enemies run out at the end of the road. Two of them are right next to one another as they run at you. Hold your breath with to go into concentration mode. This will slow them down, but incase you don't kill both of them. You can just reload to the previous checkpoint.
This is very easy. After your first saved checkpoint, where you are guided across the dirt road where the 3 guards are talking, turn around after you cross. Then, line up 2 of them and use concentration mode. I got 1 through the head, and the other in the back.