Cats who live together will often fight. Usually one cat has to be the dominant cat. Sadly for cat owners, cats do not resign themselves to being the 'underling' or accept that they live with the other cat, so they may as well just get along together. No, they insist on fighting!
The best way to ensure that blood is not spilt is to ensure that each cat has space. Each cat also needs a separate litter tray and a separate one which can be used by either cat.
Try to establish some safe hiding places for each cat, so that they can easily avoid danger. If one cat repeatedly sits in a doorway, move it. This may look innocent but is actually a feline aggressive pose, designed to control who comes in and out of the door.
If one cat appears to be the underling, ensure it has some high resting places where it can feel safe. But try not to get too hung up about it. Sometimes cats just fight to get attention. But, if it becomes very serious then you may have to talk to your vet to see if he can recommend a cat behaviourist.

 Hedgehog's answer is great and gives lots of ideas for things to do so there isn't much to add. However, I was thinking that perhaps your cats are only play fighting which is natural behavior for them when they live together. Mine love to stalk each other, and sometimes hiss and growl, but they never have hurt one another. I think its really more upsetting to us than to them. If your living quarters are over crowded that is very stressful to cats as they tend to be rather solitary creatures for the most part. I wouldn't worry about it if no one is being hurt or acting fearful all the time.

 Hi

if you see two cats fighting all you need to do is shout very loud and peircing and they will stop. (there may be hissing but this is normal)

this has worked many times on my cats.
If one of the cats is hiding i a corner and moaning just give them some water, food and stroke them.

Hope this helps you

adam (owner of three cats)