If there is a conflict between your cats and one of them seems stressed, provide additional litter boxes in locations where the anxious cat spends the majority of her time. (aspca.org)
Have additional litter boxes in each new area that it has access to. (smallanimalchanel.com)
You may also need obtain additional litter boxes for your cat so they don't have to travel far to find them. (allfelinehospital.com)