Can two cats share a litter box?

Technically yes, but it’s a setup for problems. Cornell’s guidance on litter box behavior is specific: one box per cat plus one extra, distributed across different parts of the home.[1] When two cats share, the lower-status cat often gets blocked from using the box and quietly starts eliminating elsewhere. The shared box also gets dirty twice as fast, and cats avoid dirty boxes. House soiling that develops in a shared-box household is most often resolved by adding boxes, not by training. Two cats, three boxes, in different rooms.

When to call a vet: if you’ve added boxes and one cat is still eliminating outside the box, see your vet — urinary issues commonly present this way.