introducing full grown dogs

Question
We rescued an adult female pitbull. we already own a three year old female pitbull. upon initial meeting my gf brought them dog into the house. which i realize was a mistake...the dogs immediately showed aggression over toys on our first dogs part and attention on both our parts. to reduce the risk of them hurting themselves we purchased muzzles for the pair of them .they continue to go at it we the muzzle on. it seems as though our female has grown tired of the new female but the new female seems to stare our original female down. without the muzzle and separate the new female has a wonderful temperament and seems so much like our current female..almost identical in disposition..my real question is how do i effectively train them with the muzzles without just teaching them the muzzles stop U from biting? need help?????/!!! if U need more info just ask

Answer
I'm afraid that you might not like my answer, but I want to be as honest with you as I can.  Firstly, you did make a mistake, but I'm not sure it would have made a difference had you not introduced them in your home.  The American Pit Bull Terrier is a great family dog - with humans they are seldom aggressive, and tend to be loyal and happy companions.  But, unfortunately, many of them are dog aggressive - this is genetic, and not something that you can easily train away.  You would, perhaps, have had better luck adopting a male, especially if it were a puppy age 5 months or younger.  Most adults tolerate puppies, especially of opposite gender.  Two males, of varying ages and social status might even get along, especially if one is a pup when they are put together, but I wouldn't have suggested that either - I'd much rather go with opposite genders and a gap in ages.  Females of any breed that do not like one another are more likely to cause serious damage when they fight than males are, especially if they are of similar age and status (ask any long time trainer about "bitch fights").  With this breed, you are in definite trouble if you have dogs that are staring one another down, posturing, or "going at it" repeatedly.  These are dogs that can easily cause huge vet bills, or even kill one another, so it isn't safe to have them together unmuzzled, and it's a pretty miserable life for them if they have to live with such constant stress.  It really seems unfair to your resident dog to now have to live like this, with a hated interloper in her home, and the new dog is probably not happy either.  This is a bad situation for the dogs and for you.  My advice, sad to say, is that you should consider returning the new dog to the rescue - I think she, and maybe even your dog, would be better in single dog households.  If you would like more information in general on owning multiple dogs, there's a DVD by Trish King on the subject that is very enlightening.  But, there are special considerations when you own APBT's.  Some of this behavior is genetic, and not just "how you raise them", as I'm sure you did a nice job with your dog, and it certainly isn't your fault - just how these dogs react sometimes.  
I feel badly even having to give you the advice that I did, and I certainly invite you to consult personally with a behaviorist in your area, in case there is something that I am overlooking, but this is my take on it just from what you have told me.  Let me know if you have any other questions, or if you would like help finding a behaviorist.