It sounds like an explosion waiting to happen. Are the two whippets bitches? If so, I think it would be safer if one was to go, once there is fighting, the tension will always be there. It sounds like the usually well behaved terrier just got carried away with the situation. Maybe less dogs and more daily obedience training one to one will help stabilize the situation. Good luck. Dogs fighting is scary and you cannot be there 24 hours a day to keep the peace.