Hi and welcome to TCS!
Cats are territorial, and unlike dogs they are not pack animals. They can learn to love and enjoy each other - but that's not necessarily going to happen. They may just learn to share the space.
When bringing a new cat home, it's really best to put kitty in a separate room for a few days or few weeks, depending upon how things go.
You have to let the resident kitty get used to the idea that there's another cat around that is going to share his space, and the new kitty has just been removed from what he knows and needs a little time to get oriented. Pretend you're six, your family just moved to Germany, and EVERYTHING is different. You need a little time to get used to everything.
Introducing them slowly really helps both of them.
Here's a great article: http://www.thecatsite.com/Behavior/4...cing-Cats.html
Get a room ready for your new kitty: put food, a litter box, water, maybe a scratching post or mat, and toys in there. Make sure there is a hidey spot for kitty.
I'd consider purchasing Feliway Spray. Use it liberally around the house - but not near scratching posts or litter boxes. Feliway is a synthetic hormone that mimics the "friendly" marker in cats' cheeks, and it will help both of them feel less stressed out by all the changes.
Make sure you spend time in the room with new kitty - but also make sure to give your resident kitty lots of extra love and attention. His needs take precendence.
They'll smell each other under the door. You may want to stand there with the door open a couple of inches so they can see and smell each other, but not actually interact.
Take some rags or towels and rub new kitty all over with them. Put one under your existing kitty's food dish. With the other one, put treats out for him each morning and evening. Do the same thing for new kitty with towels that smell like your resident kitty. This will help them come to associate good things with each other's scent.
Put new kitty in a carrier, and let existing kitty see and smell her. Then put him in her room and close the door. Let her out of the crate to check out the house. OR bring him into her room for 10 - 15 minutes (if they're not hissing at each other through the door after a day or two).
Keep an empty can with coins in it handy. Rattle this loudly if one attacks the other.
Remember - hissing and batting will be normal. Watch the body language though. Tails up is friendly. Tail down and very agitated and ears back is NOT good. The ears back is a signal that things are not OK. Rattle the coins only if there is true aggression.
Extend the amount of time resident kitty spends in new kitty's room each day. If things are going well - open the door. New kitty has her territory now - the room she's been in - and can flee to her safe space if necessary. Resident kitty has figured out he's going to have to share his space. Now they have to work out who's going to be alpha. Let them work this out, and use the coins only if one cat is actually going to get hurt.
And what may look like aggression to us, may, in fact, be play. Cats do wrestle.
(That's why you've got to watch the body language).
If you don't want to keep the second litter box in the separation room, move it to where you want it after a week or so that the door's been open - if new kitty is spending most of her time out and about.
You should consider adding a 3rd litter box, though. The rule of thumb is that you should have one more box than you have cats. It helps avoid any inappropriate peeing problems.
Hope this helps,