In my experience if a normally clean house-trained cat starts weeing outside the litter tray, he or she is trying to tell you that something is wrong.
If you know that both cats have happily shared their trays, that you have not changed to a different brand of cat litter, or have started washing the trays in a different brand of washing-up liquied, I am at a loss, as your vet cannot find anything wrong.
Could you possibly collect a urine sample from the other cat`?
My two are so fond of each other that when the male suddenly lost all his outer coat of fur along both flanks, and the vet could not identify the problem, that it dawned on me he had become allergic to their cat food, when his sister started bring live mice in and handing them over to him.
As I was not thrilled by mice being regularly killed and eaten in the hall, I changed their diet and lo and behold! six weeks later his fur was in perfect condition again.
I have just remembered an embarrasing happening years ago, when a visitor in her stockinged feet trod in a pool of urine beside the cat tray. While she was washing her feet and putting on clean stockings, I emptied the tray and found a hair-line crack in the base of it. So, the cat had weed in the tray, but the dampness had seeped out!
