Are there any kitties outside - either strays or neighbors' kitties who might be wandering by?
Or were there any kitties living in your house who might have peed or marked, and they're smelling that?
Be sure to clean up their pee wherever it is with an enzymatic cleaner like Nature's Miracle; otherwise, kitties will still be able to smell their pee and be attracted back to the same places.
I'm assuming kitty 2 has been checked for a UTI as well? I've never had a kitty with crystals, but I think it can be painful, in which case pain could be causing litterbox avoidance.
A few things you could try that I've learned here worked for some kitties:
- Dr. Elsey's Cat Attract litter, or the additive they make that you can sprinkle on top of regular litter.
- Putting a layer of soil over the litter.
- Taking out the litter and just putting puppy pee pads in the box.
If it's anxiety:
- A plug-in pheromone diffuser.
- A calming collar. My vet swore by NurtureCalm.
- Anti-anxiety homeopathic drops, like Bach's Rescue Remedy or HomeoPet's Anxiety Relief, in the food or water.
And, to make your life manageable, put pee pads around the box where they tend to pee.