Well, you're in a hard situation, as you well know.
Is there a chance that one or both women would understand your feelings and why you did what you did, if you came clean and spoke honestly?
Do you see any viable future in maintaining a dishonest relationship structure?
Do you think it would be better to -- like Stevie Nicks sang -- pick up the pieces and go on?
Can you imagine a way to introduce the concepts of polyamory to one or both of these women in such a way as to help them understand what's happened?
...The answers are yours, and there may be other questions you want to ask yourself. As you look for answers, keep your thoughts constructive and realistic, avoid despair, and inside yourself be as completely honest as possible.
Mistakes are made. I've made 'em. Good luck!