I think a behaviourist would be the best route to go down, they will be able to observe and let you know if it's a minor or major problem. As Dex is just reaching adolescence, it sounds to me like a battle of testosterone, and status, and Barney is asserting his authority. It would probably settle as Dex matures and status is sorted out between them, but occasionally it can escalate, and it would be sensible to get professional advice on how to manage the situations.
In the meantime. I would take all toys away, treats given to them separated. Is it all gob and gestures or do you think Barney is really attacking Dax to cause an injury?