Ok, my tests (backwards since I found a little inconsistency, removed the whetstones with 3End+10Fireresist):
Tests done completely unbuffed, Ability: Dual Strike
2x4DPS Whetstones -> 786
1x4DPS Whetstone on Mainhand -> 772
1x4DPS Whetstone on Offhand (cleansed and reapplied, didn't switch weapons) -> 772
No Whetstones -> 757
Inconsistency: When Applying the Whetstone to the offhand, I crit for 764 once, then consistently for 772. Also had this the first time applying a Whetstone to the Mainhand (this is why I did the tests once more backwards). The strange thing is that I could have sworn that my damage without Whetstones was 764, too, on the first run and it dropped to 757 on the second (backwards run) but I didn't write it down on the first run.
Overall it feels strange.
Shard: Brutwacht, BTW