Dear Barbie,
I’m so sorry to hear about your recent troubles with the younger kitty urinating in unwanted places.
Inappropriate elimination can be caused by medical conditions such as urinary infections or by behavioral problems. Sometimes, it can even be associated with a more serious medical problem such as a urinary obstruction, which is an emergency situation that requires immediate attention.
But cats also may go outside the box when they dislike their litter or if the box is dirty. They can even develop this problem due to stress from changes in their household, such as a new pet, or moving to a new home.
Obviously, if your kitty is urinating outside the box for a medical reason, he should be seen by a veterinarian as soon as possible. The “outside the box” behavior could soon cease if the cause is a medical condition.
It doesn’t sound like you’ve had any significant changes in the past six months to cause the kitty undue stress, so my best hunch is that the kitty is tired of sharing the litter box or has a medical issue.
The easiest thing to try first would be to purchase that second self-cleaning litter box so that kitty has his own place to eliminate. There is a common rule of thumb that says you should have one litter box for each cat, plus one. That would mean three litter boxes for you! But I’d say try having two first, and that may resolve the problem.
In the meantime, you probably should take the cat to the vet for a physical exam, to rule out a medical cause for the inappropriate elimination.
Best wishes for a positive outcome!