Good Morning Val and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
It is not unusual, Val, for dogs in the same home to develop subtle territory issues between them and this behavior is causing this. If niether dog is spayed, this is your first step since hormonal influences can cause this behavior.
Follow these tips:
1. Purchase a DAP Collar (Dog Appeasement Pheromone) at the pet store. This is a canine pheromone that acts as a calming agent and will decrease anxiety. Rescue Remedy is also helpful. Use this on both Bonnie and Bella.
2. Carry a coke can with pennies in it and when you see this aggression starting, shake the can loudly and say a strong NO!
3. Only have them together when supervised and feed separately for 1-2 weeks to decrease their anxiety.
4. have your vet examine Bonnie and Bella checking for a medical cause such as hormonal issues or metabolic problems that could cause this. If this is ruled out, ask your vet for an anti anxiety medication such as Clomicalm or Reconcile that will decrease the anxiety that is causing this.
5. Consider contacting a veterinary behaviorist that can help your girls past this past this.
6. TakeBonnie and Bella back to a good obedience class so you will have better control over her.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, Dr. Loretta