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It sounds like they are not very happy with one another! I suggest that you separate them unless you can watch them.
The reason why they are fighting might be unknown. If they are not spayed or neutered, this might cause some agression so you should have this done if you are able.
If there are any new stresses in the home, this can cause some discord.
There is a great homeopathic remedy that we have used with a lot of success. It's called homeopet (www.homeopet.com) and it's a natural anxiety reliever. It can get cats through the hard times.
I hope this helps!