Oh the age old question! If only there was a quick miracle answer.
It sounds like you've already let them see and smell each other, so there's no going back now to trying to gradually introduce.
Are they currently eating and drinking close to each other? Are they using the same litterbox as each other? If so, they will learn to co-exist in the same house -- although it might take some time.
3.5 years ago, I made the mistake of getting a kitten as my 16 year old cat was certainly getting on in his years. The kitten drove my old man crazy wanting to play and have fun. For 7 months, every day, the kitten asked the old man to play. And the old man responded with a growl, sometimes a hiss, and usually a raised paw. One day, after I had completely given up, I found them curled up sleeping together. They've been the best of friends ever since. We've added 3 other cats and the old man tolerates them, but only the kitten (who is now 3.5) is allowed to sleep with him, and they groom each other. It's a beautiful sight.
Back to your question. Here are some things you could try.
Take a towel (dish towl, washcloth will work too) and rub it all around one cat's face, concentrating on the cheeks and under the chin. Then, take the other cat and rub that towel all over him/her.
Do you wear perfume or cologne often? If so, take some and spray/splash it into your hands. Wait a minute or two. Then, rub your hands over both cats.
The goal with either of these methods is to get the cats to smell alike. Cats are very responsive to smell and if they smell alike and in the second example, smell like you, they may take to each other better.
Go to the pet store and pick up some Feliway -- in either the spray or the plug-in version. It's unscented to us humans. But it is a synthetic form of feline facial pheromones' and it relaxes cats. Whenever I've brought in a new cat, this has been very helpful in integrating them. It's also great for trips to the vet in the car or any stressful situation.
You may be able to find Rescue Remedy at a pet store or online. It's an herbal remedy you can either spray into the air or add to cats' water. Again, it reduces stress and may help them get along better.
Try some toys with Catnip in them spread around the place. You can also buy catnip and just put it into some old socks that you tie in the leg part. Cats don't know the difference between expensive toys and socks believe it or not. They're not materialistic like us! That will relax them and give them something to play with. Of course, not all cats like catnip .. but it's worth a shot.
Also, don't separate them as long as they're not injuring each other. Some growling, hissing, spitting, and even raised paws is to be expected. A few little bats isn't a problem. If you see one cat biting the other, it's time to separate them as that's when injuries can occur. Otherwise, you can allow them to work it out amongst themselves. Remember, the cat who has been there longest has just had his/her territory invaded so it can take some time.
If they are getting too aggressive, then it's time to separate and re-introduce very very slowly and gradually. If you can put one in a separate room, it's best. Usually the new one! Then, allow them to play foot soccer under the door to get used to each other's smells. For an hour or so a day, switch them and let the older cat go into the room, while the newer cat stays outside. This will again reinforce being in each other's areas and smells!
Good luck! All our 5 cats get along really well now. 4 of them play all day and cuddle with each other. The old man doesn't play anymore but he often watches.