I wish I had some good news for you…but sadly, I don't. We have a divided household here as well. It is not uncommon for the younger bitch to decide somewhere between 6 mos and 2 years that it is time for her to move up...and the older bitch decides she would rather kill her than let that happen.
It is really unlikely IMO that you could get these two together again, but if there is a chance (especially since there hasn't been a large blood letting), you might look for a behaviorist that is familiar and practiced with dog/dog aggression. Positive reinforcement training only please, or it may make the aggression worse. It takes a lot of management, and a huge time investment. But, you are already doing a lot of management keeping them separated.
One of my concerns would be what if your daughter accidentally lets them together, and one kills the other? I wouldn't want my child to have to bear that
I really am sorry, I wish I had better suggestions for you.