this isn't fading puppy syndrome. there are no seizures in that as a general rule. The puppies simple "fade" slowly and die with no apparent cause. Seizures in 2 puppies that young that result in death says "toxin" to me. Get mom and the remaining pups to the vet ASAP. I would also have a necropsy done on the 2 pups to try to determine cause of death. and if something genetic is the cause then you need to contact every owner of every puppy you have ever sold from this ***** and let them know.