I think you should bring it up with bf#1. A surprising number of guys are okay with sharing a girlfriend as long as they know what's going on and they feel comfortable with who the other boyfriend is, i.e. not drug addicted, dirty, promiscuous, etc. I've been in that position a couple of times in the past, before I'd ever heard the term polyamory. In one case her boyfriend wanted to date another girl and she figured if he could, so could she. In another, she rekindled a relationship with an ex. while we were dating. In both cases I knew all about the situation and they let me know my wishes counted for something in the relationship even if I couldn't tell them what to do. In one case I knew the other guy fairly well, and in the other I never met him. I don't think it matters much. (my current situation involves her relationship with her high school girlfriend, but that's a little off subject)
Whatever you do, be above board with everything and if one wants you to tell him details of your relationship with the other, understand that it's probably his way of dealing with insecurity and heading off jealous feelings and since it's part of the life of a woman he loves I think it's ok to talk about it with him.
The more I learn about other peoples' relationships the more I find that it's not all that uncommon, people just don't talk about it much.
Last edited by Stevenjaguar; 11-20-2012 at 03:41 PM.