The answer is simple...just doing it is the hard part.
Tell them both the truth. I wouldn't necessarily do this at the same time with them sitting together or anything insane like that, but they both deserve the truth and you deserve not to keep up with your lies.
Since one already knows you are interested in pursuing a poly relationship, hopefully this declaration will get her to tell you what she wants, or doesn't. For the other, it's a coin toss, either she will forgive you or not, either she will understand poly or not (although that inculdes a not yet sometimes).
Polyamory is based on communication and HONESTY. If you continue with both of these relationships without the other knowing, it is not Polyamory. It is cheating.
Life is about the journey and not the destination,
so what better way to know life
than to wander all the roads and paths set before you.