In Deus Ex: Human Revolution for the PC, PS3, and Xbox 360, there is an augment you can purchase that will allow you to perform a double takedown on enemies. The double takedown is a great move when you can perform it, because you get bonus XP compared to taking them down individually.
In order to perform a double takedown, you need to purchase the Reflex Booster augment on the right side of the augment map, which costs two praxis points.
With Reflex Booster purchased, you simply perform a regular takedown like you normally would, and if there are two enemies within close enough vicinity it will automatically happen.