Twin

Source Core Rulebook pg. 283 2.0
These weapons are used as a pair, complementing each other. When you attack with a twin weapon, you add a circumstance bonus to the damage roll equal to the weapon’s number of damage dice if you have previously attacked with a different weapon of the same type this turn. The weapons must be of the same type to benefit from this trait, but they don’t need to have the same runes.

Weapons

Butterfly Sword, Buugeng, Exquisite Sword Cane, Exquisite Sword Cane Sheath, Fire Poi, Hook Sword, Sawtooth Saber, Throwing Knife, Wind and Fire Wheel