Over the years, I have had many queries with regard to feline behavior. Having multiple cats can be challenging, for sure. Cats can be quite mercurial, and knowing what is influencing any negative behaviors is key.
Felines are very primal creatures. They normally have territories in the wild, and defend them ferociously. At times, spraying/marking/peeing could be a territory marking strategy. I think that the idea of keeping only specific areas available to them indoors is an excellent tool, especially if your cats are allowed to go outdoors as well. This will do two things - allow them to have shelter when they need it, but at the same time the cats will learn that this space is theirs, and that they need to respect the space to keep it comfortable and peaceful. My vet gave me the idea of only giving my cat, Silver one room to have as his, after a bout of urinating on furniture and my clothes. Silver caught on after only a few short days and 'knock on wood', years later he minds his manners! Creating specific areas for eating/drinking, resting, hiding, playing is important... places that are 'tucked away', so to speak and areas that are 'up high'. Most cats have definite preferences on where they like to claim as their space - up on top of a cabinet, a cozy space under a stairway, in a basket of old towels. In my many years of experience with cats I noticed that some cats will usually rest in a old box under the basement sink, and others will perch on top of the lockers/tall storage containers - and yet another cat would always be parked inside of a cabinet in the back corner. Very habitual, cats are! Most importantly, cats generally prefer to have their eating/drinking area as far away from the litter box as possible.
Sometimes when cats urinate outside of the litter box, it is a sign of a urinary tract infection. Interestingly enough, cats usually identify the litter box as the source of the pain/discomfort that they may be feeling. The behavior may even persist after they've seen a vet and the infection has cleared up - cats have an impeccable memory.
Pee in the litter box -------> feel pain ------> pee in other place as an alternative
Some cats will not use a litter box if another cat has already used it. Having one litter box per cat is the standard, and it is good to have them on opposite sides of the room. Felines are very fussy about so many things, especially cleanliness.
Above all, it is a long held misconception that cats are independent creatures who do not need a lot of attention. Quite the contrary! Although the behavioral habits of cats differ from other animals, this in no way means that cats do not need as much attention from their people. Cats love routine. They thrive with one-on-one time, playtime, grooming, treats - they are very social creatures. Most cats over the age of 1 year LOVE dried cat nip - try some, especially during these cold winter months when things can get a little dull. Fresh cat nip can even be planted outside in the garden (I plant it near the door and Silver seeks it out immediately when he goes outside), and it does very well in this climate zone. Cats need exercise - a few minutes of playtime with a cat friendly toy and your participation can help keep your cats' weight in check. Excess pounds attribute to disease and respiratory problems like asthma.
Spend a little bit of extra time with your furry friend today, they deserve it. It will do you good, too.