we used to have two boxes and my cats used one for pee and the other for poop. i have heard many cats prefer to have separate boxes for each.

also when my cat that passed away last december was getting older she wouldn't poop in the box anymore she was so old we didn't try and change what she was doing just kept paper and stuff around box to make mess easier to clean up and kept her as comfortable and happy as possible. the veterinarian said that a lot of cat's when they get older will find the position to poop painful and then associate pain with the litter box so avoid the box. but if op you already talked to a vet that may not be case for you.

my current cat i have had for a long time and he is so picky about only using a box that smells like he has used it that he once held it for 36 hours at the veterinarians. i have never known a cat so picky about having his box and i have no idea why he is so picky. he prefers to go outside if that is an option rather than use a box at all but if he needs a box it better be his box and only his box.