How to properly bond metal junction box with 6 AWG conductors
First off, why is the ground #6? Seems big unless the circuit is over 60A.

Personally, as Tester commented, I'd use a double lug screwed to the box via a machine thread screw, that is if you can find one small enough. The one Tester linked is quite large for the application. Smaller ones might be hard to find though.
This one is the smallest I could find: 
http://www.cesco.com/b2c/product/Burndy-K2A25U-Two-Conductor-Terminal-Lug/159535

Another alternative is to splice the two grounds to a third tail and connect that to a single small lug screwed to the box. 
http://www.cesco.com/b2c/product/Blackburn-Elastimold-L70-Type-L-Slot-Screw/3978