Thank you for the reply.
The very first thing I would do it to have a urinalysis performed on Missy. Even though there is likely a behavior component to it, there may be an underlying medical issue at play as well and if it is not identified and corrected, you will never get the behavior under control. The condition called feline idiopathic cystitis (FIC) can occur intermittently in cats with higher levels of anxiety and the stress or anxiety may not even be something we recognize, having to share living quarters with another cat can be one of them. On top of that, forcing her to stay outside when she may not want, can only add to her stress. FIC can cause inflammation of the bladder which may or may not result in visible blood in the urine, but it is usually visible microscopically. This can lead to discomfort and some cats going through it will spray or otherwise eliminate in inappropriate areas. It is thought that they can develop an aversion to their litter box as they associate the litter box with pain. The problem can wax and wane on it's own and present very intermittently, so it would be best to check a urine sample close to the time that she is eliminating inappropriately.
Once a medical condition is eliminated, or if she is found to be free of disease, then you can focus on getting her back to the litterbox. The goal is to make the litter boxes the most attractive place in the house and the other spots less attractive. The general rule of thumb is to have one more litter pan than you have cats. Since there are two cats, you should have at least 3 litter pans in the house if not more. You can have 2 of them side by side if need be. Some cats develop a preference of one for stool and one for urine. Maybe try one covered and one uncovered to find her preference. Clumping litters seem to be preferred by most cats because it is soft. It also allows you to keep it cleaner as there shouldn't be a wet bottom. You may need to experiment with different litters, different types of boxes (short sided, tall sided, etc) and different amount of litter in the boxes. You must scoop daily! Cat's are very texture specific so look at the places she is choosing to go. If it is a carpet, you may want to put a piece of scrap carpeting in a litter box for her and slowly add more litter as she starts to use it. If she is toileting outside as well, if she using soil, if so, maybe try soil in one box. Some cats, mine included, prefer to urinate in an empty litter pan with no litter at all. There is also a litter box additive called Cat Attract® which works very well. PRECIOUS CAT LITTERS LINK You may have already tried these, but they are suggestions that have helped many of my clients. Litter pan hygiene is key. This is to help with the problem of the inappropriate urine and stools that is more likely a substrate preference or litter box aversion. Urinating on clothes or shoes may be a marking behavior and more a sign of anxiety.
When dealing with anxiety in a multi cat house, it is best to try to identify the source of the anxiety. In Missy's case, it may be being forced to share her living space with another cat or it may be due to cats or other animals she is encountering outside. If the indoor cats do not like to live with one another we have to strive to provide an environment where all cats are able to find a place where they feel "safe". That begins by creating an environment of "plenty." There should be plenty of litter boxes, food bowls, climbing towers, toys and resting areas in multiple locations. All the litter boxes and food bowls cannot be clumped all in one place because that forces the cats together, something they may not want to do.
If that is not possible due to the layout of your home, another option is to create a time sharing plan. One cat is out in the house for a bit, then put into a room and other is then allowed out.
Next, you want to make the other spots unattractive or unavailable, confinement is one way. Maybe keep her from the places it is occurring on the clothes and shoes, if possible, especially when you are not home, unless that is where the litter pans are. Enzyme products are needed to break down the odor causing components of the urine. There are a few products that work very well to do that: Nature's Miracle® (link here), Anti-icky poo® (link here) or Zero odor® (link here) Deterrents are needed after that. Carpet runners (with plastic spikes) turned upside down work great for large areas. There are also "scat mats" which give off a small electrical charge. These are not harmful to her. Another option is an indoor invisible fence to keep her out of certain areas of the house. They are very effective. LINK HERE
There are small devices that can keep her from going through a doorway as well LINK HERE. Also there are motion detector type products that work very well and are inexpensive.
One thing I would suggest is a product called Feliway®. LINK HERE Feliway is a synthetic pheromone that mimics the natural chemicals that a cat secretes. It creates a comforting, reassuring feeling that has a calming effect. It has been shown to decrease urine marking and spraying within 30 days with a 95% success rate. It is also calming to cats in stressful situations such as transport, hospitalization, veterinarian visits, boarding, new environments, pets or people. Feliway® is a product that can be sprayed or used as a room diffuser. It can be purchased through veterinarians and pet stores. I would definitely put a few of these around the house.
There are drugs that can be tried, but I would try the other things first. Even if you have to resort to drug therapy for a while, these are changes that should be incorporated at the same time. Anti-anxiety meds such as Prozac have been shown to effectively reduce urine spraying.
This is a hard subject to cover adequately in a few paragraphs. I am including a link that may give you a few more ideas. Work with your veterinarian who should have experience with this also as this is a very common problem, but can be solved in many cases.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.
My posted replies are for general education only and not meant as a diagnosis. Only after a thorough veterinary examination can a diagnosis for your pet be made and specific treatments be advised or medications be prescribed.