If the animals are small, introduce them to each other will be easier. Small kittens are easier to adapt to the emergence of a new friend. Launch the kitten into a room with another animal and watch their reaction. Let them sniff each other and get acquainted. Prepare for two separate bowls and troughs, even if they will eat from one dish. Give the kittens the same amount of attention, let none of them feels deprived. If you notice that they are aggressive against each other, or one is configured for combat, you can drink a sedative for cats. If you will be attentive to the kittens during this period, you will soon notice how they have fun together and play together.
It's more complicated if you need to make friends with adult cats, as they already have a formed character. Some animals find it hard to accept the fact that their site will live someone else's. For starters let cats sniff each other. Better if the new beast will be in carrying, and the door will be closed. Perhaps you will immediately see that the cats start to hiss at each other. If this does not happen, we can try to release a new cat. Just watch closely the reaction of both. If you see the first signs of aggression, then divide animals.
If from the first minutes, the cats are not found understanding, then placed them in different rooms. In the room where it will live a new beast, check the trough and a toilet. Let it first grow roots in one room. And my old cat share a litter or other thing, from which emanates the smell of a competitor. Let it gradually gets used to it. With the same purpose several times a day change animal rooms.
Pay enough attention to one and the second animal. So none of them had any reason to be jealous.
Can pull between the iron mesh, so the cats are unable to get through it, but could each other well to see. You can also two sides of this partition to put the bowl of food. It will also help the cats to get used to joint existence.
When you notice that the cats have stopped hissing and endeavor to fight, then the grid can be removed. But at the same time in the next couple of weeks should not leave the animals behind. Leaving for work or business, it is better to dissolve them on different rooms. If within a few weeks you see that the animals behave well and friendly, we can assume that your task in their acquaintance is made.