Take them both to the vet and insist on exploring this in depth. One of them is trying to tell you something, and it's not, "I have decided to become an evil, unloveable kitty."
My best buddy cat developed diabetes without my realizing it. He tried his best to get to the litter box, but with the amount of liquid that was gushing through him he also ended up peeing his brains out in other places in the house before I picked up on the clues -- like HUGE clumps in the clumping litter. An easygoing cat can be managed with diabetes. Unfortunately Spook wasn't one of those and I decided he had to be put down immediately. I might have gotten the first insulin shot into him, but my arms would have been tattered shred if I tried a second.