Hello and thanks for researching this important subject. I have a pitbull and an amstaff and just LOVE them to bits! These can be the gentlest dogs around and you obviously are very committed to giving them good care.
These links outline several reasons for something called Sudden Aggression Syndrome that can occur in dogs:
http://www.petplace.com/dogs/medical-causes-of-aggression-in-dogs/page1.aspx
http://www.canlyme.com/dogagg.html
http://critterweb.com/btneuro/soa.htm
Since this problem has come on quickly and is getting worse, I do recommend that you by-pass all regular veterinary offices and find a Veterinary Behaviorist who will work with you all to find the cause (if possible) and to get these girls back together.
You are wise to enforce a separation at this time.
The behaviorists at the University of Pennsylvania can be reached for appointment by calling: 610.925.6220
Specialists at Cornell University Veterinary Hospital can be reached for appointment by calling: 607 253-3060 (option 2)
If these centers are not near enough to home, the scheduling desk or behavior department may be able to refer you to someone reliable nearby.
Thanks for not giving up too soon!
Your concern has touched me--if you have follow-up questions, or need to clarify your case, please let me know right away! Your "Accept" helps me reach out to others!