That is just awful. I'm so sorry. First thing I would do is see if there is a veterinary neurologist near you. A specialist may be able to help in ways a regular vet can't.
Did the cats come from different breeders? If not, I'd start asking the breeder if there is a hereditary component. If from different breeders, it's anybody's guess what's going on. But you might think about environmental factors that could be causing the issue -- contaminants of some sort, parasites, household cleaning products. Could be any one of a million factors. Is you cat exposed to areas where there are raccoons? Raccoons carry a roundworm called Baylisascaris procyonis. Cats can pick it up from walking through contaminated soil/grass. The worm migrates through the optic nerve and brain. It's not a common infection as far as I know, but then, I don't know if many vets even think about it as a possibility so it may be viewed as uncommon because no one tests for it.
Did your vet do a necropsy on your first cat?