This is actually slightly hard to get because there don't appear to be that many opportunities where two guards are actually close enough to trigger the option. You need to put one Praxis point into dual takedown before you can get it.
The location I was able to get it at was when I entered the XNG Building, I went to the right and into the North bathroom, found a vent, and followed it around to another room. Around some boxes were two guards with their backs to me; I was able to sneak up on them and get the dual takedown option and unlock the achievement.