For a cat, sharing their important resources (such as food, litter trays, beds, human attention etc.) with another cat, especially if they haven’t grown up together, can be really stressful.
Cats can often see other members of their species as ‘competitors’ rather than ‘allies’. Signs of tension between cats can be quite subtle and so owners can easily not pick up on them.
Owners need to provide, and carefully place, all their key resources to help reduce tension and conflict between cats and encourage them to live together in harmony.