tleete's page

"There's no need for an official answer because, as has been stated numerous times in this thread, the two are written to be mutually exclusive. There is nothing in Double Slice that states anything about giving you extra damage from Power Attack, just Strength, nor is there anything implying that it should/is meant to."

...except the preamble to the description of Power Attack, which says "You can make exceptionally deadly melee attacks by sacrificing accuracy for _strength_." (emphasis mine). Both Power Attack and Double Slice are about strength - the extra strength you put into a Power Attack, and your ability to apply equal strength to your off-hand attacks with Double Slice. I can understand the "it doesn't specifically say you can, so you can't" reasoning being used here, but a little common sense suggests that double slice ought to apply to power attack as well.

Oh, and the preamble for Double Slice says "Your off-hand weapon while dual-wielding strikes with greater _power_." (emphasis mine, again). Yup, I could see how something called _power_ attack wouldn't be influenced by something that increases the _power_ of your off hand... ;)