How does Power Stance work? Is it more effective than simply dual-wielding?
Power Stance works by holding down Y or Triangle depending on your console. You need two weapons of the same type in each hand (two clubs, two daggers, two short swords, two great swords) and 1.5x the stats required to wield the weapons. You gain increased damage and a different move set at the cost of increased stamina expenditure and obviously you weigh more which further impacts your stamina usage.
In short it does more damage at the cost of more stamina expended per attack. In my opinion if you're doing to dual wield, power stance is a no brainer.