The description for the dagger-based Puncture skill states that it doesn’t do additional damage with double-grip (nor with critical hits to the head), but the damage range is 15-35 and cornered marksmen sometimes do more than 40 damage with Puncture (listed as Puncture in the battle log) and have a double-grip icon on their tooltip (I also thought I saw Puncture doing critical head damage in the past, but that may have just been double-grip).