Fighting!!

Question
Please help.  I have a huge problem that has just come up between my two cats.  Both of them are 2 years old and female.  They have been raised together since they were both between 6-7 weeks old.  They are not siblings though.  My problem is this.....just recently they have started to fight (ALL the time.)  One of them usually does the hissing, growling and pinning the ears back while the other one frantically whips her tail around and stares at the other.  I have no idea how all of this started, or how to resolve it.  Please help soon.  I couldn't deal with having to chose to keep one over the other.

Answer
Sarah,

I am assuming they are both spayed.  If not, that would be the first oreder of business.

My bet is that some strange cat came around and, after seeing this cat, the two girls got into a fight/flight situation, and, when cats do this, they often forget who each other are!  

Normally, this subsides in a few days to a couple of weeks. I would ignore the discussions and keep out of it.  Human intervention will prolong the readjustment.  Neither cat will really hurt the other.  There may be hissing, spitting, posturing, "fighting", even loose fur.  However, cat skin is very, very tough, and, even though it may appear to be mortal combat, nothing generally really happens!

I would leave it alone and let it resolve in its own way.  Humans do not understand how cats inter-relate and should just leave things alone.

Best regards... Norm.