The op was correct, in that it doubles all of your damage bonuses and combines them into one hit.
It does not convert all of your damage to the same dmg type as your weapon.
Calculate how much divine dmg you deal, and go to stonevale and auto one of those divine ghosts. Your hit should be exactly what you calculated it to be. Now use double attack, and your hit should be twice that.
My mistake then! I was told tgat skills like sneaky converted all damage to that of your weapon. Assumed double was tge same