I don't have much in the way of advice that hasn't been already been given but I can say that I do feel for you. My husband and I had a roommate that had a female Rottweiler and at that time my female Lab, Natala was still alive. The two dogs could not be within sniffing distance of each other, without one going crazy. For a while, we managed to keep them separate but one day when I came back from a walk with Natala, my roommate had his dog loose(stupid stupid thing to do when you have no control over your dog) and the Rott attacked Natala. Mind you, Natala was no spring chicken and she had bad spondylosis and hip dysplasia and the Rotti out weighed her by 30 pounds but that girl ripped into the Rott like the odds were on her side. I honestly feel that if my husband hadn't clobbered the Rotti over the head with a piece of lumber to seperate them, one of them would have ended up dead. After that day, the guy was told that he had to find somewhere else to keep his dog otherwise he could pack up and leave that day. I was taking no more chances because you never know if and when it could happen again.
Whatever happens, I hope you find the best solution for you and your two girls. I can't even imagine how difficult the decision to rehome one of them would be but you seem like you know what needs to be done. Best of luck to you.