The Offhand Doublestrike description is like this: Your offhand melee attacks gain a percent chance to strike twice:
But offhand doublestrike is not working as described. It only happens when you normally do a mainhand doublestrike.
For example, I have 25% mainhand doublestrike chance and 20% mainhande doublestrike chance. Only when my mainhand doublestriked, the offhand may doublestrike(35% chance). So the real Offhand Doublestrike Chance is 20%x35%=7%.
I used my tempest ranger to test this both on Lam and live:
Attack: 100 times
Main-hand doublestrike: 9 times
Off-hand doublestrike: 3 times
This really disappointed me. And due to this mechanism, the capstone of Tempest--Dervish--which add you 25% Offhand Doublestrike chance is far underpowered.