Probably a good idea to broach the subject with boyfriend first. If he's totally opposed, there's your hang-up and there's no sense in getting second boyfriend in potentia excited about the idea only to have boyfriend shoot it down.
Bring up the subject in a subtle way, maybe mention polyamory in the context of entertainment. Read up on Polyamory: Married and Dating and ask if he's heard of it and he'll probably drop a few hints as to his feelings on the subject. If its visceral negativity, that's where you need to work on. If he's positive on the idea, then you can start a real conversation about what you want.
Also be prepared that boyfriend in potentia may not want that kind of relationship even if you and boyfriend do.
I am as direct as a T-Rex with 'roid rage and about as subtle. It isn't intended to cause upset, I just prefer to talk plain. There are plenty of other people here who do the nice, polite thing much better than I can. I'm what you'd call a "problem dinner guest."