Sisters who dont want to seperate. what do we do?

I have two nieces, who are sisters, and have been raised by my mother and I. They are extremely close and do just about everything together. They share clothes, a room, basically their lives revolve around one another. They have also attended the same school since Pre-K and are in the same grade (they are only a few months apart). Both are smart girls but one is a little more studious and has better grades so the chances of them getting into the same high school is slim. We have tried to prepare them for the possibility of being seperated but that only induces tears and fits of rage. They are so upset and have vowed not to be seperated. How can we help them realize that seperation is not a bad thing and that they can make it through this? I am afraid that the seperation anxiety is so strong that they may do something drastic in order to stay together.