When one of my cats was just a rescue kitten, my older female cat wasn't too fond of her. They get along a bit better now that they're both adults, but when the little one was just a baby...

The older one used to literally scare the shit out of her. She would just surprise her from around the corner or something, and the baby would piss in place, diarrhea in place, or run to the side and blast a turd trail in her wake. She would sometimes run under the couch where the big one didn't fit and just drag a snail trail of feces under there.

She's still very alert when she needs to use the box, but there are enough boxes for everyone to share, and a big house where everyone has their favorite spots. No more accidents. They have a yard for the first time, and they get along very well out there-- neutral territory.