Most conflict in multi-cat households comes down to resource competition rather than cats simply "not getting along" as a fixed personality trait. Getting the environment right resolves a surprising amount of tension that owners often assume is unfixable.
The resource rule of thumb
For litter boxes specifically: one per cat, plus one extra, placed in different locations rather than lined up together (a row of boxes in one spot functions, socially, more like a single resource a confident cat can guard). The same logic extends to food and water — multiple, separated feeding stations reduce the low-grade tension of one cat controlling access to the only food source.
Vertical space matters more than floor space
Cats resolve a lot of social tension through vertical territory rather than direct confrontation — a cat that can retreat to a high perch has an alternative to a standoff. Cat trees, shelves, and window perches meaningfully reduce floor-level conflict, especially in smaller homes where horizontal space is limited.
Introducing a new cat properly takes longer than most people expect
A rushed introduction is one of the most common causes of lasting inter-cat tension. The standard approach: full separation with scent swapping (swap bedding between the cats' spaces) for several days to a week, then supervised, brief visual contact through a barrier (a cracked door or baby gate), gradually increasing duration over one to several weeks before full unsupervised access. Cats introduced too fast often develop a wary relationship that's much harder to repair later than it would have been to prevent with a slower start.
Reading early warning signs
Staring (a prolonged, fixed stare is a real threat display in cat body language, not neutral curiosity), blocking access to resources, and one cat consistently avoiding a room are all signs the current setup isn't working, even without obvious fighting. These are worth addressing early with more resources or a slower reintroduction, rather than waiting for open conflict.
When it's not working
Some cat pairings genuinely don't reach more than a tolerant coexistence, and that can be an acceptable outcome — not every pair of cats needs to be close companions, as long as each cat has enough separate resources and space to avoid chronic stress. If you're seeing signs of chronic stress (over-grooming, house-soiling outside the litter box, appetite changes) despite good resource setup, that's worth discussing with your vet, since chronic stress has real physical health effects, not just behavioral ones.