I have done a test in the Test Dojo.

The description text is:
Dervish: You gain +2 Dexterity. You now have a +25% chance to doublestrike with you off-hand when dual wielding.

I take the PTWF feat, now my main-hand double strike chance is 10%(without any items, just enhancement+feat) and off-hand doublestrike chance should be 35%(25% from Dervish, 10% from PTWF), but the test result is:
Attack: 100 times
Main-hand doublestrike: 9 times
Off-hand doublestrike: 3 times
And, off-hand doublestrike never occurs independently, it always occurs along with main-hand doublestrike, just like its additive.

So the real effect is far different from description:
When you make a normal doublestrike(main-hand doublestrike), there is a chance that you will make a off-hand double strike.

I want to know whether this is WAI?

If yes, then Dervish is far underpowered. A tempest ranger with decent equipment will have a 15-25% main-hand doublestrike chance, so the real off-hand doublestrike chance will be only 3.75-8.75%. IIRC, the original design of Dervish is "full stat mod to damage with off-hand", then the current design is far weaker than that.

If no, please fix them..