|Barf and Belch|
It was revealed in the show that the left head's name is Barf and the right is Belch, with Ruffnut apparently commanding Barf while Tuffnut controls Belch.
In the episode In Dragons We Trust, Barf and Belch are framed by Mildew of the crime of stealing all the boots of the village during "boot night," sleeping in a bed of boots. They are then proven innocent by the comparison of their tracks and the tracks at the scene, noting how their tracks are deeper.
In Twinsanity, Barf and Belch briefly went rogue after Ruffnut and Tuffnut's conflict with each other reached a point where they were issuing contradictory orders to the head they controlled, the Zippleback roaming around the village just as renegade Viking chief Dagur the Deranged arrived to discuss a treaty. Wishing to sign the treaty in dragon's blood, Dagger captured and nearly decapitated Barf and Belch, but the twins were able to reconcile in time to help their dragon escape, the other dragons faking an attack on the training ring to conceal Berk's new trained dragons in order to avoid provoking war.
In View to a Skrill Part 1, when Tuff asked Ruff to make Barf produce gas, Ruff answers "With pleasure!", hinting that they love the explosion.