Unicorn. I love the term, personally, because I think it's appropriate for what it describes. Yes, there are plenty of beautiful, available, bisexual women. That doesn't mean any of them will be a perfect fit for both people at the same time, in terms of a long-lasting emotional relationship.
But from what I've heard, there actually are a surprising number of women willing to just jump into a sexual threesome with another couple. I'm not one of them, most women on this forum aren't, but that's because this is polyamory.com and not threeways4u.com