This book confirmed that by stating that it is best to have opposite sex dogs I think that pointing to gender as the focus for behaviorial/D2D/aggression issues is simplistic.
TrainingĀ (conditioning) is also only a part of it.
I do agree some aggression issues can't be fixed easily.
Bad genetics are bad genetics.
I had to euthanize a beloved older female as she was teaching my younger dog to be dog aggressive.
I had let her think she had the highest ranking member in my pack.
So this was opposite sexed dogs but due to the psychology of the dogs, it was not a healthy pairing.
It depends more on the temperments or personalities of the dogs, place in the pack....I would say than just the sex or spayed or neutered state.
No book can do as well as a consultation with canine psychologist --- dogs have to be observed and analyzed in the unique situations with the people and dogs...all the pack members!
The best tact to take with aggression is to ferret out a good behaviorist!
I would see a vet to make sure that there isn't an underlying medical problem.