If you got a second kitten later than the first, after a while, it will not probably be happy. After all, it gets used to; it is the one who gets all love of its master and so on. Most likely, it will defend its territory from a stranger, and will not let him get to his bowl, tray, cottage/couch. Therefore, if you take a second kitten, then make sure that it has all its own: it individual bowls, another tray and, if necessary, couch. It is necessary to ensure that it does not invade the territory of the first cat, the risk of being bitten and scratched.
Generally, it would be better if you can settle a new pet in another room, so that the first cat have no access. Over time, you can try to give them some time to be in the same room, for example, start to feed them in the same room, but put the bowls in different corners. With each passing day the bowl should be gradually shifted closer so that the kittens will get used to each other with time, however, it can happen that the attempts will fail.
If you brought both kittens in your house at the same time, but they have not seen each other in the first place it is necessary that they do this. When meeting, the presence of the owner is required, anything can go wrong. Let kittens sniff each other. In any case intervene them, do not try to get someone to come closer to another, or even to try on the first day put them to sleep next to each other. If, during the first meeting, the kittens have not started to hiss, bite and scratch, it is not a sign that after a few hours they will eat from the same bowl and sleep in the same house. It is possible, that their sympathy stops at that and they are indifferent to each other and allow being around each other. It is important that master always pay equal time and attention both kittens, otherwise they may start to believe that he loves much one and the other less.
It may happen, that even when meeting the kittens begin to behave aggressively - they can begin to hiss at each other, bite or scratch. Such behavior often occurs in cases when both kittens are of the same sex. If only one kitten shows aggression, you can try to calm it down - as soon as it starts to hiss on the other, or tries to bite it, drizzle it with water from a spray bottle. If both cats behave so, then you can try this method: You can pull net in the doorway. These kittens will see each other, but they cannot fight. Most likely, in a couple of days they get used to each other.
If none of this helps, and a month later, they are feud, then the only thing you can do is to make sure they do not see each other at all. Settle them in different rooms and give them their own bowls.