Hello, and Welcome!
Talk with them. Choose which one to begin with, and just say what you feel, what you're thinking and feeling....
If you're not able to do this, go to a close friend other than these two and talk with him/her about the situation. Maybe this friend can help you get prepared to open up with these two. Choose a friend that is most likely to sympathize or empathize with you. It won't be any good to seek supportiveness from someone who thinks poly is "wrong," or who isn't pretty open-minded.