Getting intimate with either or both of these girls, since they have already said they would not want you to have feelings for another if you were with them, would damage your friendship with one of both of them. If you can keep them both as close friends by not being intimate, them stick with that. Just inform any future potential partners, that these girls will be in you life.
I'm new to this whole poly thing too and I find it pretty amazing. Continue to educate these friends on what poly is so they can understand you better. Who knows, they may change their mind somewhere down the road or maybe not.