There are 3 ways to double fire these guns, one occurs when a single trigger gun has a malfunction, then there is harping the triggers, normally done unintentionally, and lastly you can fire both barrels simultaneously on purpose. When you harp the triggers your finger slips past the front trigger in recoil and fires the second barrel. This presents no hardship to the shooter, and although there is but a single "Bang!" and the muzzle rises higher in recoil but there is no greater recoil sensation because the two shots occur as separate events.
In the event that you intend to purposely double your gun, tape up your fingers and wear a glove, as they are sure to get hammered by both the trigger guard and the front trigger. Damage to the gun is unlikely, but a stock failure is not out of the question, particularly if the charge is heavy and the wrist of the stock slight. Such a stock failure at the wrist could result in a significant facial injury and an injury to the firing hand.
All in all I think it is a bad idea just from the stand point of purposely inflicting a massive flinch trigger to your subconcious, never mind the injury potential even without a stock failure. I recall an article Warren Page wrote in which he talked about the trauma he suffered when a single trigger 10 bore doubled on him. Page was a world class bench-rest and skeet competitor, a wildcatter, an international big game hunter, and a rifle crank of the most deprived type. He was not squeamish about recoil in the least, but that experience badly damaged his shoulder and required years, not months, of retraining to get beyond the flinch he developed as a result.
so it would give you a sore shoulder but this kind of recoil is not unheard of.....also a .577 t-rex generates 200 ft lbs of recoil
