From what I remember from the last double XP weekends the additional boosts are counted BEFORE the doubling. So 10% guild boost plus 25% consumable boost = 135% x 2 = 270%.
I may be wrong but I thought Bioware confirmed this previously.
That is correct for regular kills and missions. When you add the legacy perks for Warzones, Class missions, Flashpoints, Space and Exploration it goes up to 330% with the added 30% X 2
I know it's insane!