Leave them loose only when you are there supervising. You really do not want to come home to a bloodbath.
It sounds like female-female aggression or re-directed aggression. Dogs are fence fighting and can't get to the dog on the other side, and turns and grabs the one available.
Bitches are often worse than dogs, and it does not really matter that they are spayed.
RIP Arwen, CD RN CGC
RIP Whitney, RN CGC
Jenna, RN CGC & Babs, CD RA CGC HIC (not AKC)
Heidi, RA CGC & Tori, RN CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear