Unfortunately, when litter mates fight it tends to only go downhill.
liter mate aggression
There are dozens of threads on this. Most breeders will not sell littermates together, especially females, for this reason... google "littermate syndrome".
Spay them if they're not already, and keep them separate. Frequently the advice given is to re-home one... neither will flourish while both are there, competing with each other, and unless constant management is practiced it could get very, very ugly.
How were they raised? Was time away from each other emphasized?
What do you mean by what is allowed?