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Hello, my name is***** and I'm happy to help answer your question about Priest and Bandit. I'm sorry to hear that they are eliminating outside of the litterbox- that is probably the most frustrating thing a cat owner can go through. Give me a few minutes and I will prepare some information about inappropriate elimination.
Whenever cats begin eliminating inappropriately, we must distinguish between behavioral and medical causes. Examples of medical conditions that can cause inappropriate elimination include urinary tract infections, bladder or kidney stones, kidney disease, neuromuscular disease, and various conditions affecting the gastrointestinal tract resulting in diarrhea or increased urgency to defecate. These medical causes can be screened for/evaluated by your veterinarian if you bring Priest and Bandit in for complete physical exams. They may recommend blood work (to evaluate internal organ function) and analysis of Bandit's urine and Priest's stool.
Once we have determined that there is no underlying medical cause, we are left with behavioral causes of inappropriate elimination. Examples of behavioral causes include fear or association of a negative experience with the current litterboxes, competition or inter-cat aggression, and stress due to a new family member or other change in environment. It sounds like you take excellent care of your litterboxes so I left dirty or undefilled boxes off the list. :)
You mentioned three litterboxes located in the garage. Would it be an option to move one of the boxes into the bedroom or living room where the cats are eliminating inappropriately? I know its not fun having a litterbox right there in your bedroom but it's arguably more fun than cleaning cat pee and poop off your bed regularly. It could be that the cats don't like going all the way out to the garage to pee or it could also be that one cat scared the other cat while they were going potty out there and now they don't want to go back. Whatever the reason, moving a litterbox to a new location is a good place to start. You can also experiment with changing the litter type, as again they can develop aversions to the litter itself and refuse to use it.
Do you have any further questions or concerns?
I wish you and your cats the best!