The definitive answer Mac, as usual. Couldn't have said it better.

Actually, I personally have no idea how I trigger a double. When stalking in close on buffalo, or 'jumping' boars, I know I've shot only because cartridges are ejected when I open the gun, and the quarry is down! ...and that's how it should be.

The best way to practice double-triggers is to wander the rabbit-warrens with a 2-trigger SxS shotgun, keep shooting it till you are no longer aware of triggering.