This achievement is awarded for destroying two enemy tanks with one shot.
This is by far easiest when playing as an artillery and requires a great deal of luck.
Look for groups of light/medium tanks clustered together and slowly follow them with the target receptacle. If you are lucky they will collide together (this happened for me when one tank was heavily damaged and reversed into the following tank.) If you seize the moment and aim for the middle you should manage to hit both at the same time.
This is heavily dependent on luck based on the positioning of the shot, the armour of the tanks you are firing at and the positioning of the tanks.
Edit: Apologies - The video has now expired.