Praise or toss treats to reward your cats when you see them interacting in a friendly manner. Try pheromones. You can purchase a product that mimics a natural cat odor (which humans can't smell), that may reduce tensions. Use a diffuser while the aggression issue is being resolved.
The average time frame is a few weeks but it could take months for the cats to learn to tolerate each other. Maintain a separate litter box and feeding area for the new cat in his room for a few weeks. It is highly recommended to have multiple resources for multiple cats.
Let your current cat smell the towel. Do the same for the new cat. Don't be alarmed by hissing or growling. These are normal reactions.
Introducing too quickly with little preparation will often lead to cats feeling threatened and scared, which increases the chance of aggressive behaviour being shown. Once cats feel this way about each other, it can be extremely difficult to change their minds.
By having both cats experience something positive (a meal or yummy snack) while they are nearby, they can learn to form positive associations with each other. If either cat is growling or hissing at the other through the door, donot put them together anytime soon; take things very slowly and continue feeding this way.
Plan B: Re-introduction
Put both cats in separate but adjacent rooms and provide them with their own bowls, litter boxes, cat trees, and private you-time. Start by feeding them with their bowls placed on either side of the door. The distance to the door will be determined by how reactive your cats are to each other.
Separate your cats in different rooms for several days or weeks, with separate beds, bowls and litter boxes. This way they can hear and smell each other, but don't have to interact. Place the cats' food bowls on opposite sides of a closed door.
Cats that have set up separate territories in the house and don't really get along will: Chase or run away from one another and avoid contact. You may notice that one cat consistently leaves the room when one enters. Watch each other intently and may hiss or spit when they meet.
Territorial Behavior
If one or both cats begin feeling territorial about their favorite lounging spot, their litter box, or their food bowls, this can cause fights. If you notice fights that seem to come from one or more of these things, you're going to need to do some separation.
Cats with a strong link will frequently show clear indicators that they regard themselves to be members of the same social group. Grooming each other, sleeping, or lying next to each other are examples of these indicators. They may regularly greet each other by touching noses or making a little meow as they pass.
Don't be alarmed; it's normal for cats to hiss at something new or something they don't understand. Hissing is a distance-increasing behavior. Simply put, it's a warning saying, “Please back off, and do not come any closer.”
Cats are solitary, territorial creatures by nature so don't force siblings together. It may take weeks or months for a cat to adjust to their new lifestyle. Cats introduced too quickly may fight and never learn to get along. Gradual introductions are essential for long-term brotherly/sisterly love.
Give the cats a reason to like each other.
Providing them with something they love when the other cat is around or when they see the other cat. Typically, this involves giving the cats food or a treat! Play time, as well as grooming (assuming your cat likes to be brushed), can also lead to positive associations.
Give Your Cat Personal Space
To correct this, give your cat a place to call its own again. This may mean moving the new pet's feeding station to another room or giving your cat a new perch where it can observe the family undisturbed.
However, if your cat shows hostility to other cats, or hisses, growls, or marks territory if they enter your yard or garden, then obtaining another cat is probably not a good idea.
Fortunately, neutered and spayed cats are much more likely to get on well together than intact adult cats. Generally, female cats tend to be more accepting of new cats into their home. They are more likely to accept another female cat than a male cat.
The younger they are when introduced, the greater the chances of their getting on amicably most of the time. After about two years of age, acceptance of another cat can be a bit random, but they will very rarely totally reject another cat in the long-term.
The introduction must be gradual. Following the initial introduction, it can take a very long time for a relationship to grow. It takes most cats eight to 12 months to develop a friendship with a new cat. Although some cats certainly become close friends, others never do.
If one or both of the cats are intent on chasing and can't be redirected into another activity, it's time to take a step back. Work your way back through the introduction steps, really focusing on building a relationship through food and mutual play at the door of the new cat or kitten's acclimation room.
Never introduce a cat or kitten by confining one or both to a cat carrier – they do not provide any opportunity for retreat. For kitten introductions, a large crate can be used in a room, positioned in the corner and partially covered with a blanket to provide an area for the kitten to be out of sight.