Are there double sided servos?
Yes, They are sold as 'robotic' servos. Look in the Hobbyking or towerhobbies catalogues.

I got mine as part of a camera 'pan and tilt' bracket, but you can get them individually. One side drives, the other is often just a hub and bearing.

Not all 'robotic' servos have this feature. Sometimes the term 'robotic' simply means metal gears and high torque. 

Look at the individual descriptions, specifications and pictures in the catalogues.