It sounds like you have already kind of talked about it with her? Why not see if she could say something to your boyfriend so that he knows she is interested? Either when you're around so you can all discuss together or she can tell him she's been thinking about it and thinks you might be interested if he was and you two should talk kind of thing...
I do think it is important to be able to bring up big issues BEFORE getting into this kind of relationship, though. If you're not able to tell him that you're interested in this woman, how are you supposed to be able to bring up other things that are important to you? Like if you need more personal space or more time just you and your girlfriend or more time just you and your boyfriend? You really need to be able to talk completely freely with both people so you won't get stuck in a situation you're not comfortable with.