From Halopedia, the Halo wiki
Double Fire: R,R,X
Quickly press the Right Trigger, then as soon as you have your trigger released, quickly squeeze it again and this time hold. Quickly, after the controller has registered the start of the holding, press X in order to have the two shots burst out at the same time or rapidly follow each other.
Timing is crucial, as there is about 1/10th of a second to do so and within that second the pace of the shots is also determined. You will know it's successful when you see no reloading animation, instead, having a full magazine again with six bullets less.
You can also cancel the actual reloading by the Cancel Reload, but more useful is to Double fire and then instantly throw a grenade to cancel the reloading as this also seems to be the quickest way to get damage across a map or to an opponent.