i would say maybe it is a one off because your one of your bitches is in season and as they get older if they are around the same age they do have the odd scraps for pecking order.
I would say your first steps are working out who is top bitch
have them both spayed
also feed the top bitch first even with tip bits, along with petting them always the top bitch first.
I would watch them around toys, sticks or any type of food as this sets some dogs off.
If they were really fighting or it was a vicious fight they would have ripped into each other mainly going for the neck, not the legs and you would have had a lot of trouble parting them, i think it was maybe a one of due to one of them being in season and scrapping over a sick.
I have 7 dogs
6 females and 1 male
5 unspayed females
1 spayed female
and 1 uncastrated male
i have dealt with my fair share of fighting dogs luckly not with these but with past dogs