There are two questions.
HB#1 = your current girl
HB#2 = your current girl's friend
1. How good of friends are they, as the previous poster mentioned. If they are only acquaintances, move on to the next step
2. Is HB#2 in to you? If so, and they are not best friends or anything (the reason you are on step 2 right now) Maybe throw a hint out there to her... " I am kind of "with" HB#1, but I think you are fucking awesome. Id love to find out if you are as interesting as you are cute/sexy/etc. , but what do we do about HB#1? If she's into you, HB#1 now becomes and issue for both of you - freeing you from guilt and accusation
I don't have any experiences with 3-somes (but hope to change that soon)
If you think there is a chance to make this happen, try the Dual-Induction massage routine. I tried to post a link, but was disallowed. Just do a google search, first link.
-ub