Originally Posted by Voxx

Stones never stack - as stated above - and Windfury is a much greater dps increase than a sharpening stone, even though it applies to both weapons.

Edit: Updated the original post to reflect 6.25% as the Expertise "cap." This is done in light of the idea that 6.25% is close enough to the cap as we can determine at the present time without grossly overshooting the cap. The only combinations that could easily overshoot this "cap" with the current itemization would be Humans/Orcs with Weapon Mastery and some additional item with Expertise (like T6 Belt).