This is such a sad situation. You seem to genuinely love these women and I wish you didn't have to feel this pain.
The closest to your situation I've ever come was dating two men in my twenties, one my ex fiance and one I fell in love with at work. The ex fiance "A" didn't know the details, just that I had broken the engagement because I didn't feel I was ready to settle down with one person yet. The coworker "J" knew all the details as he came to the relationship secondly and it was just easier to be honest with him as it caused A too much pain to know everything. After over a year of going back and forth and each wanting a decision from me and J even trying to date other women, I lost them both. J could no longer take my indecision and left. A wanted someone completely mono and decided he could never trust me and I let him go because he never would have left on his own and never would have accepted me as I am. It hurt. It devastated me. I ended up in a psych ward for a couple of weeks because, not knowing what poly was, I felt dirty and wrong and like all I did was cause pain.
Years later, I am married to a wonderful man, "F", who loves me as I am and I have had a relationship with a woman during this marriage. It ended, sadly, but it strengthened my relationship with my husband and we are very happy together. I'm no longer interested in other men in a physical way.
I guess what I am trying to say is that you may have to make a decision and it may hurt all of you. The women, as much as they love you and you love them, may simply be monogomous by nature. As much as they want YOUR happiness, your happiness may cause them nothing but pain. This is something you'll have to discuss with them. Are they willing to try poly just to make you happy or do they each think this is truly a lifestyle they can live? Are they interested in their own other loves and how do you feel about that? Could opening themselves to others help with the fears and the second choice feelings?
As the other poster said, have they met? Perhaps if they do and can see eachother as the real human beings they are, and not the idealized "other woman" it may help calm their fears. If a friendship were to bloom between them it could help all of you with communication and reassurance. Funny question: Are they bisexual? If a relationship developed between them in a romantic way you may all get the loving relationship and feelings of completeness you're looking for. But don't press that issue. It's just a thought.
I applaud you for your openness from the very beginning and your constant attempts at keeping these two loves happy. It is so much more than I was ever able to do. I hope you get all you want in these relationships. But I want you to know that, even if you must let one or both go, there is someone(s) else out there for all of you. In time, life does go on. But not if anyone is forcing themselves into staying in an unhappy situation.