Welcome to the boards!
The obvious question is this: Have you spoken with them about it? Everything hinges on you being forthright and honest about what you want. Without you bringing up the topic and discussing it, nothing will happen. Two women who are accustomed to mono relationships are not going to suddenly get the idea that they want to share a man, even one with whom they have some relationship history. If you want to be involved with both of them, then you have to discuss it with both of them.