give them time to adjust to each other. they won't become friends over night. keep a barrier up between them, and limit their interaction, until they are more comfortable with each other.

If they're both spayed/neutered then I'd recommend Feliway pheromone collars/plug ins. Also, for safety, make sure you're cutting their nails often. Sometimes playing with one with a laser, string toy, etc. while the other watches behind a barrier then vice versa can help as well. Lots of attention and treats to both. Always have a sprayer bottle with water and something that can make a startling sound (like waving around a piece of aluminum foil) can help break it up long enough to separate them at the least. Best of luck! Your veterinarian may also have some helpful advice :)