And yes, too many ppl give up because it's too hard. It's easy to love a dog. Not so much to be the leader. That's not always fun.
...this I can relate to.
I was one of the those that "gave up". It was one of the hardest decisions we've ever made - and also one of the best.
"giving up" is sometimes the most responsible, and generous thing you can do
for the dogs.
In our case, we didn't raise both dogs, The older one was ours from puppy-hood, but we adopted a 1yr-old bulldog mix that came with "issues" when our pup was ~ 5yrs old. They did ok on their meet & greets, and the rescue group never suggested he had an aggressive bone in his body. But, after a week with us, he started acting out.
So we learned the hard way by being dumped in the deep end, that to be the "leader" can be a very hard and lonely place to be. As "leader", we were not able to spoil either dog - no cuddling, no couch time, no one allowed on the bed with us - ever. If we let the submissive dog on the bed, the other one would attack out of jealousy. If we let the "bully" up for some love after being good all day, as soon as the submissive dog was in sight - he'd launch himself at him to keep him away... which just got him in trouble. So... we missed out on part of our favorite aspects of dog companionship, all so we could be "leaders". No fun.
We did use many different levels of containment to keep both dogs safe, but there were the inevitable scuffs and scrapes when all looked well... until it wasn't.
After 3 years of getting 98% of the issues resolved (the bully was down to only attacking the older dog about 1/month, normally while he was SLEEPING), we decided to rehome the "new" dog. EVERYONE was happier when the stress of trying to keep everyone coexisting peacefully was lifted.
The bully immediately took to his new family and was able to relax without another dog around.
The older dog was able to live out his remaining time (less than one year) in a stress-free home, and reverted to his happy-go-lucky great-dane-ish puppy antics. I do worry how much of his demise (cancer) could have been avoided if his immune system had not been lowered due to living in stress for 3 years. It's hard to get a good night's sleep when you expect to be attacked at any time.
SO... my advice:
DO seek professional advice and see if there's something you're missing.
DO try NILF and see if this is something you can work out of the system. Being two girls reaching maturity, you may be able to convince them that they have NO NEED to fight for "top bitch" honors, as that is YOUR position. They might test boundaries for a month, or 6, or 12, and then happily go back to being besties.
but... they might not.
...and/or keeping the peace might be more work than the rewards are worth...
Only you will know when enough is enough.
Good luck and stay strong!