Don’t overlook the very important first step of visiting the veterinarian if your cat is pooping outside of the box. I have lived with a cat who had inflammatory bowel disease and know how much pain he must’ve felt when his intestines starting cramping. I also have a number of clients who have cats with intestinal problems. Getting your cat diagnosed and on appropriate medication (and in some cases, prescription formula food) as soon as possible will be most important.
There are some cats who don’t like to defecate in the same area used for urination. For some cats it may be that urination has more territorial connection or it may just be a quirky feline instinct. Regardless, a simple solution you can offer is to make another box available for defecation. Don’t place the box right next to the original box or it’ll just be regarded as one big box and your cat will still not poop in it. In many cases you can put the second box in the same room (depending on the size of the room), but in other cases, you’ll have to locate the second box elsewhere. Your cat will certainly let you know when the location pleases her.
It typically takes a cat a bit longer to defecate than urinate. In a multicat household where there is even the smallest amount of tension, it may be too stressful for a cat to hang out in the litter box long enough to poop. If the box is covered, wedged in a corner or hidden in a closet, this truly reduces the cat’s escape potential. She may feel it’s safer to poop in another location that allows her to have a better view in case an opponent is coming. The location she chooses may also give her a better opportunity to get out of there more safely.
The solution in this case may be to provide uncovered boxes and to make sure there are enough boxes located through the house. Don’t place them in hidden, cramped areas that may cause your cat to feel trapped or confined. In many cases, all you’ll have to do is remove the lid from the box. Covered boxes are often to small and low for a cat to feel she can comfortably perch on the litter for defecation. They also limit the cat’s escape to just one way in and out. Should another cat come by, the one who is in the litter box can be vulnerable to an ambush.
Some cats, for whatever reason only they seem to know, have a substrate preference when it comes to the feel of the litter for defecation versus urination. Perhaps it has something to do with the amount of time they spend in that perching position for pooping. If you think that might be the case, offer another litter box with a litter that has a different texture. In general, cats prefer a soft, sandy texture when it comes to the litter substrate.