I really feel for you. Because whether they have been in your life two months, or twenty years, it’s painful and it hurts to lose them like this. It is absolutely devastating. I want you to know that you are not alone with this experience. It's the happiness and love that comes along when someone new enters your life, and then having that stolen away from you before you can even figure out what has happened. I see the pain and anguish on the faces of people who have been through this. I'm sorry it happened to you again.
For those reading who don’t know, FIP (feline infectious peritonitis) is one of the worst, devastating viral diseases your cat can be afflicted with. It begins as a very common, normal infection called corona virus. This is a normal, every day virus that can easily spread among cats as frequently as common cold can spread among people. But this virus can change abnormally into another virus called FIP. Especially if you are genetically predisposed for this to happen. FIP can effect many vital parts of the body in a bad way, and we still have no good way of treating or diagnosing it. There is also no single test that tells you if if they have FIP. There is some new research being done for treatment options (i.e. Polyprenyl Immunostimulant), but it is nonetheless very difficult for both cats and people to go through.
So this brings us to Minky and Heidi. If we want to make sure they are not heavily shedding corona virus to any new cats that could arrive, we can do that. There is both a test and a method (PCR fecal testing over time). And this could help put your mind at ease, which is of value. It is also true that Minky and Heidi could test positive for coronavirus, but they were not the ones who spread it to your third cat. I say this because corona virus is commonly found in indoor cats, and in some circumstances up to 90% of healthy cats living in a household have been found to have corona virus. Transmission of FIP directly from cat to cat is extremely rare. Also, in most homes where a pet has passed on from FIP, there is little risk to any new cat’s introduced into the same home. You can increase your chances of not running into FIP also by adopting an older cat, of genetic lines or bloodlines not predisposed to FIP.
I believe that you could adopt another, third cat, without it suffering from that virus.