2014-07-18 : If multiple effects modify how damage is dealt, the player being dealt damage or the controller of the permanent being dealt damage chooses the order to apply the effects. For example, Dictate of the Twin Gods says, “If a source would deal damage to a permanent or player, it deals double that damage to that permanent or player instead.” Suppose the equipped creature would be dealt 3 damage while Dictate of the Twin Gods is on the battlefield and you control two creatures. You can either (a) prevent 2 damage first and then let Dictate of the Twin Gods’s effect apply, for a result of the equipped creature being dealt 2 damage, or (b) double the damage to 6 and then prevent 2 damage, for a result of the creature being dealt 4 damage.

2014-07-18 : To determine how much damage to prevent, use the number of creatures you control when damage would be dealt, including the equipped creature. Notably, if damage would be dealt to multiple creatures at the same time, you’ll count the number of creatures you control before any of that damage is dealt.