An adult cat is extremely reluctant to accept other cats in the house. Our friends left their cat for 2 weeks while they were away. All these weeks my cat hissed at the cat and even tried to throw herself. The cat sat in one room and was afraid to walk around the house. It takes more time to adapt. Although some do not accept it. They can live in the same house, pretend to ignore the second cat, but periodically they will show that they are more important here.