[Kromm]

Canonically, Power Blow counts for the next small-a "attack" (not for the next maneuver, turn, etc.). Such an attack could easily involve a two-handed weapon, a Drop Kick using both feet, a Two-Handed Punch, or the like. It would be decidedly odd if one limb had twice the Strength of the other in those cases . . . you'd think that would result in less than double ST for the attack. Thus, I'm inclined to say that Power Blow affects ST for any group of attacks that occur simultaneously, which includes not only Dual-Weapon Attack but also All-Out Attack (Double) or Extra Attack when used to strike with two different limbs at the same time. All-Out Attack (Double) with a single weapon, Altered Time Rate, Extra Attack (Multi-Strike), Rapid Strike, etc. produce what are explicitly successive blows, not simultaneous ones, so they're out of luck.