This first became evident after the battle against the Red Death, in which both Ruffnut and Tuffnut learned to command their respective dragon heads. Now, the four of them have formed a strong bond. Ruffnut prefers to ride on Barf and Tuffnut tends to ride on Belch.

Who rides barf?

Dragon Team: Ruffnut & Tuffnut

Ruffnut & Tuffnut are Barf & Belch's trainers and riders.

Which head is barf and which is 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.