Unfortunately, dogs pick their friends, just like we do, and there really are some dogs that hate one another and cannot be trusted together. Any time a human intercedes in a dog fight, there is a risk of a redirected bite. Better to stick a chair between them, or blow an air horn, or something like that, than to use hands or get too close. Sometimes, it's better to rehome one of the dogs than to make them live in abject terror, or to wait for a huge vet bill to happen. This is precisely why we advise most people to get dogs of opposite gender. The fights between females can get very ugly indeed, and can result in one of them dying if it escalates. Males are more likely to fight ritualistically. IMO, better not to EVER let your guard down, and that might include strict separation, rather than just watching - so that a human doesn't get sucked into the fray again.
Regional Director for Massachusetts, International Positive Dog Training Association
Director, SeniorCare Pawsitive Connections Program
AKC CGC Evaluator #3669
Therapy Dogs, Inc. Tester/Observer
Sioux, CGC, TDInc.
Maska, CGC, TDInc.
Sequoyah, CGC
Fergie, Retired Lap Dog, Age 19
Dancer, CGC, TDInc. (1989-2006) #1 Heart Dog
"If you talk to the animals they will talk with you and you will know each other. If you do not talk to them you will not know them and what you do not know, you will fear. What one fears, one destroys." - Chief Dan George
"The fidelity of a dog is a precious gift demanding no less binding moral responsibilities than the friendship of a human being. The bond with a true dog is as lasting as the ties of this earth can ever be." ~ Konrad Lorenz