Cobra Lash appears to only use the main hand weapon.

When I have a gemini claw in my main hand (and a different weapon in my off hand) I gain mana on every attack. When the gemini claw is in the off hand I never gain mana. When My main hand has fire damage and my off hand does not, enemies ignite normally, but when only my offhand has fire damage enemies never ignite.

Visually it looks normal, the tool tip has both main and off hand stats, and it will alternate between the claw and dagger animations when wielding one of each. However it appears to only actually use the main hand weapon.