In my opinion you should cut all ties with the one who cheated on you try to get out of the friend zone with the other. The reason why she wants to maintain friendship could be that she does not want to be hurt and if you are sincere to her she might accept dating you.
It ain't no sense being with the other girl who is cheating on u. Even though u r in love wit her. That's how your feelings get really hurt and your heart get broken badly. Spend more time wit the one who wanna b friends and u will never know she might start liking u more than a friend when she gets to know the real u.