[cifani090]

Ive seen on headphones two balanced plugs, but than look at the Audio GD Roc there's only one balanced hole for headphones. Where does the other headphone plug go?????

 

[nikongod]

Depending on the amp it can be done with 1 plug. 
 
Electrostatic headphones connectors and 4-pin XLR's are examples.

 

[krmathis]

It all depends, as there are different plugs used:
* 2 x 3-pin XLR
* 1 x 4-pin XLR
* 1 x 5 or 6-pin for electrostatics.

 

[Currawong]

Regular headphones with one plug have a contact point for L+, R+ and the ground/signal return shared between left and right.  Balanced connections don't have the L and R sharing the signal return (and, as well, the signal return isn't shared with the ground, but that's another matter). This means only 4 pins/contact points are required: L+, L-, R+, R-.  Since the original balanced headphone amp, made by Headroom, was two separate amps in one box, they used two plugs, choosing the regular 3-pin XLR connectors for it, but with the gender reversed. The third "ground" pin isn't used by headphones.  The AKG K1000, however, used a single 4-pin XLR instead, though it was usually connected to the speaker terminals of an amp, but there was a special headphone amp made for them if I recall correctly, with a single 4-pin XLR socket. Since then, manufacturers and DIYers have used one or another system of 1x 4-pin or 2x 3-pin plugs, or both.
 
Balanced, in actual fact, has nothing to do with the headphones or plugs and really has to do with the topology of the amp driving them.