Hime, as kestrel said, just take it one step at a time. What you are thinking about is not strictly poly, as Oz has pointed out, as that would involve some emotional attachment.

Something I would suggest that you seriously consider before you get into it though is will this actually violate your agreement with your husband. My guess is that your anxiety actually stems from the thought that you might end up caring about your friend more deeply, or even loosing her over this. Discuss this with your husband, ask her to discuss it with hers, and then get together and discuss all of the ramifications as two couples. If it ends up being a poly relationship with both couples involved, then there are even more possibilities that may present themselves. If you end up deciding to fore go the temptation in order not to risk the friendship, that is also good. But think about the possible repercussions and have frank and open discussions about them with all the parties involved.