I'm so sorry to hear about your dogs. The seizures are due to neurological damage which was brought on by vaccinations. Several of the ingredients in vaccines are very potent neurotoxins & carcinogens. You should target healing the brain damage via organic CBD oil for dogs or perhaps concentrated Yucca extract. You can halt vaccines (they're poison & completely ineffective) and get rabies state exemption forms from a holistic or integrative veterinarian near you. I also suggest feeding your dogs an organic, whole food diet. You are what you eat. If you must feed kibble, try Orijen.
My chi is also on phenobarbital for seizures, also of unknown origin. He had small ones now and then after I adopted him, and then one day had one that went on for over 45 min, that is when the vet said it was time to put him on the phenoB. He got it twice a day at first, but seemed so sleepy and dull that I just started giving it once a day. He still has the occasional short seizure - but he's more awake and happy most of the time, so I think the one pill a day is better
I have not tried, but research 'yucca' as it is suppose to prevent seizures. Amazon; yucca intensive, liquid form.