My wife just gave me a friendly reminder that the stones were in her
bladder, not kidneys.  How could I forget?

After our dog passed away, I adopted a kitten for our 10-year-old cat (he seemed
to miss the dog; he wuld wander around the house meowing as if calling to her).
In the past, my expereince with multiple-cat households was that the cats always
got along, would play ThunderCats together, would sleep and cuddle together,
etc., so I was quite surprised to discover that Old Cat *hated* New Cat.

Even now, after several months, Old Cat just barely tolerates New Cat; hen New
Cat tries to play, Old Cat just hisses and swats New Cats' nose. Old Cat also
dislikes sharing his litter box with New Cat; he would rather sh.t on the floor
on the new couch than lower himself to use the same litterbox as New Cat.

Right now I have them separated in different parts of the house, each with their
own litter box and food/water dishes. It's a pain in the a.s, but it's an even
bigger PITA cleaning up cat poo every day. :(

Anyway, not to dissuade you, but I wish someone had told me that this sort of
reaction in Old Cat was possible; I certainly would have been much more careful
about the adoption, probably doing a "trial run" for a week or two to make sure
they would get along well before finalizing things.

I hope things turn out better for you!