My cat had FIP and because the treatment isn't medically approved, my vet couldn't even really talk about it, let alone prescribe a treatment, since it could put her license at risk. But she gave me enough hints so that I could do my own research.
There are groups out there that help cats with FIP. I was able to get the needed medication and treat my cat. It's been 6 months now since he's been cured and I'm so happy I was able to save his life.
The medication is called GS-441524 and is similar to Remdesivir. Gilead refuses to get GS-44 certified for vet use because it's so close to Remdesivir that humans could use it to treat COVID at a lower cost. Fuck big pharma and their greed.
I'm glad that you were able to save those 2 cats! My little Potato was 7 months old when he was diagnosed and it was pretty bad. It was either I get him the treatment or he would need to get euthanized. I had just recently lost my 13 year old cat and could not go through another loss so soon. I'm really glad that I helped him.