I got a kitten when my first cat was a full-fledged adult, and First Cat was EXTREMELY unhappy about the new arrival. Tried to attack the kitten and took a chunk out of my arm in the process. Just totally flipped out.
Today, she likes the newer arrival, and I would say the cats are friendly if not besties. (Note: it’s been two years.)
Definitely separate them for now, as others have said. Then, if the tests for FIV/FeLV come back negative, try:
1) Scent-swapping. Rub a face towel against one cat’s cheeks and leave it in the middle of the other cat’s room. Do the same with the second cat. Keep doing this until the cats are not avoiding or attacking the towel that smells like the other one.
2) Allogrooming. Get a goat hair brush or other brush with soft bristles and brush both cats with it. This creates a shared smell and mimics the effects of living in a colony together.
3) Food separate, treats together. Cats need to feel secure about their resources, and I didn’t make much headway when I was trying to feed them together. But joint treat sessions have helped. Find treats they both love and dispense them to both at the same time (on opposite sides of a baby gate if you have to separate them to keep the peace).
If the tests don’t come back negative, I’d suggest keeping them in separate parts of the house. But hopefully that’s not an issue.
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