Cat owners often plan to get another cat after getting one, so the two can play together, have a companion, and perhaps even have better personalities. However, cats are very territorial animals, so we need to consider more carefully how to get the older cat to accept the new one.
First, the new cat should ideally be young and small, so that the existing cat won't feel threatened, thus taking the first step towards harmonious coexistence. 
Secondly, it's best to avoid having both cats be male. Even if two kittens get along well, you can't be careless. Once both male cats mature, they may still exhibit aggressive behavior towards each other. Therefore, it's best to avoid this combination when choosing a new cat.
The two cats shouldn't have too different personalities. If the new cat is too lively, it can make the introverted older cat nervous, potentially leading to aggressive behavior.
A newly brought-home cat needs time to get used to its environment and owner. Therefore, avoid letting the two cats interact immediately. Isolate them for one to two weeks to prevent the new cat from catching any diseases. During this period, the owner should spend time with the new cat daily to help it accept you and lay the foundation for future interactions.
Now it's time to start the interaction, but the owner shouldn't rush things; take it slow. First, give the other cat something with the new cat's scent to allow it to gradually become familiar with the new cat and stop resisting. Once the new cat is comfortable with its owner, you can bring it out of the room to meet the owner. Increase the time spent together gradually, observing whether they attack each other. If the initial meeting doesn't go smoothly, you can bring the new cat back inside and replace the initial door barrier with a screen window or similar partition that allows them to meet but not attack. Also, increase interaction between the two cats during feeding time, using food to reduce hostility. After that, the two cats will gradually get along well.
