Just be straightforward. If they're real friends they won't drop you, even if they're not interested. I would start by asking if they know anything about open relationships, explaining that you're in one now, asking if they've ever considered it for themselves and then, if things seem right from there, just put it all on the table, tell them that you really like them and might consider being more than friends if they're interested too.
Be warned, dating both members of a couple can be really tricky. Don't go into anything with a set idea of how it has to look. Maybe you'll develop a strong connection with one but not so much the other. That can still be a very viable situation, as long as you can maintain a decent friendship with the other.
Good advice here: www.morethantwo.com/coupledating.html
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.