Introducing kittens to each other is simpler than introducing grown cats, so get it out of the way while they're young. Without a strong sense of territory or raging hormones, kittens are generally happy to make new friends, so take it slow and let them feel each other out.
Separate the kittens while they become aware of each other. One way to safely do this is to keep them in separate rooms with a closed door between them. Between their senses of smell and sound, they'll know that another kitten is around, and they'll likely sniff around the door and stick their paws underneath here and there. While adult cats may need this to go on for days, kittens are much more receptive to introductions, and you can open up the door after just a few hours of separation.
Supervise while they feel each other out. While not as inherently aggressive toward newcomers as adult cats, kittens may still huff and puff a little bit before they let their guards down. If they hiss a little here and there, or arch their backs, let them. And remember, kittens are playful. If they wrestle a little bit, that's one thing, but if they start to fight for real, it's time to split them up for a few hours.
Give your kittens separate spaces within the same room, so that they can learn to coexist without encroaching on one another's space. For example, give them separate food and water dishes on opposite sides of the room. Provide them with separate but identical pet beds and litter boxes, and make sure that the room has enough toys and play areas to go around.
Pay equal attention to your kittens, so that they don't feel threatened by each other. Take time to snuggle and play with each kitten separately, so that they don't resort to competing for your affection.
Watch over them while they interact. Unlike older cats, kittens may only need a day or so to get used to each other, so don't worry about limiting their playtime.
Tom Ryan is a freelance writer, editor and English tutor. He graduated from the University of Pittsburgh with a degree in English writing, and has also worked as an arts and entertainment reporter with "The Pitt News" and a public relations and advertising copywriter with the Carnegie Library of Pittsburgh.