First of all, forgive me for being so rude and not saying this at the start -- hello and welcome!
It's good to hear that you guys have thought about this scenario and are committed to honoring whatever love comes your way, even if it doesn't take the exact form you are seeking.
It is certainly not impossible for you to find a woman who's into you both, but couples often set out with these very unrealistic ideas of who such a woman needs to be and how it will all work. Thus the unicorn term, since people think they are looking for a real woman but are actually looking for an idealized imaginary woman to full a role, and are shocked when reality doesn't match with fantasy. Here are some good thoughts from the perspective of the potential new woman, that may be helpful for you: http://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.