So, my apologies first of all, but let me clarify. You like one girl with a bf, and one girl without one. The girl with a bf dumped hers for you. The girl who was originally single likes you as well, but you don't feel that way towards that girl to the same extent as the girl with the bf.
That is what I interpreted from what you said. Now, I would like to say that you are, in my strong opinion, going in the wrong direction. Girls who dump their boyfriends for others have the tendency, in general, not to have a great tenacity for long relationships. I know you most likely have strong feelings for this girl with the exbf, but I believe this girl who was single, and who liked you, is the wiser choice. It's your choice though. Go one direction, and hope to god that you never have to turn back.