Wow this situation sounds very similar to what my bf has dealt with in the last few years. I think I can talk from the perspective of your ladies because I am with a man who is poly and I am not.
My bf first discovered his poly nature about 6 years ago and in that time he had many different scenarios similar to the one you are in: he would have a gf...then meet another one, the first one would be hurt but agreed to let him do whatever because she loved him, then the second one would be hurt because he wouldn't leave the first one, both girls wanting his happiness as well as their own, everyone in pain and confused......
He got through it. You can too. Not to cast a dim light on your future, but my bf is no longer with any of the girls he had these scenarios with (I would estimate about 6-8 girls involved at various times in a 6 year period). Some of them he no longer speaks to (or rather they no longer speak to him - he loves them all still and would like to be friends), BUT some of them are his closest friends now.
Many of them were truly monogamous and could not change that. Some of them were poly themselves but wanted different things. Then you have me, who started off VERY MUCH monogamous (the thought of my bf with others tormented me and I still feel a bit queasy, but it's getting better every day), and after literally MONTHS of pain and anguish, arguments, anger, confusion, etc etc etc....
I am glad to say that I am leaning toward being poly myself now. I am open to starting my own second relationships, but not pushing towards that as a goal. I had to work through all this myself, I did a HUGE amount of self-discovery and analysis. Your girls will have to as well. Be there for them, and if they are willing to try it, be compassionate towards them. My bf had the patience of a saint when dealing with my bitter and psycho shenanigans. I love him even more for that. He really stuck with me.
My bf tried to be mono with me, he realized it was not making him happy so he moved out. We remained a couple, only now he continues to date other women. He has not yet fallen in love with anyone new and has not been physical with anyone, but I have faith in myself to be ok with that when it happens and move on with my own life. I love my bf so very much and even though I will have to share him, and we have very little time spent together since he moved out (we are both extremely busy) I enjoy what we do have.
It sounds to me like you are perhaps young (in high school? college? I will assume high school since you talk about going to the same schools) and love during your teens/twenties is always firey and die hard. Love can be all consuming and exhaust you. Simmer down, and know that even though we feel we could never live without a person in our lives, we CAN. I can remember many times when I felt that way about someone. And when they were gone, I felt I would myself die. But I am still here and I am fine.
Not trying to steal your post but I felt an explanation of my situation might help give you hope that things will work themselves out. Like my bf, you may lose them all. The choice is theirs in a way. You love them. You want them both. It is up to them whether they stay with you through that or not.
Perhaps you might tell them this: that it is their decision. They are the ones who need to work through their emotions, their doubts, their pain and find the truth in the situation. You, yourself, cannot turn off your love for these girls because eventually you will end up "cheating" like you have already done. That does no one any good. The point of being openly poly with them is to keep things honest - do not hide your feelings. If you hide your love for one girl from another girl, in a way that is a form of cheating.
It is a painful process. You will get through it and so will they.