basically you're going to have to tell your co-worker that her flirting with you is bothering you. put it in a nice way or so. she might not understand at first, but if you don't want her to continue what she is doing, you're going to have to be straight up with her and then talk to her about it. she might take it the wrong way but whatever works.
secondly, i doubt that you have a chance with the second girl right now. being that she is already is a relationship, why would she want to start another one? honestly, if i were in a relationship and someone tried to steal her away, i'd get pretty pissed off. i think you should chill on this girl because is something is meant to happen then it will. don't waste your time waiting for her because something better may actually indeed come along. raverboy
...this is just my perspective on the situation...