If I may ask, why does the female have to be single?
Unicorns are called such because of their rarity - mythical creatures. Therefore my first piece of advice would be to widen your search criteria to include any female not far off your age who is willing to sleep with you two - a male/female couple.
There are plenty of girls who are in open relationships, open marriages, polyamorous family units, and other such non-monogamous structures that would allow her the freedom - and, most importantly, the inclination - to seek experiences with other couples on a solitary basis.
My other piece of advice, that I've seen come up quite often in relation to so-called Unicorns (and so-called Unicorn hunters), is to remember that this woman is also a person. She has her own feelings, desires, fears, needs, etc. She isn't just a piece of meat to satisfy you and your partners kink/fetish for a couple of hours. With that in mind, treat her as a real, fully feeling, emotional person - before, during and after the sexual episode.
Finally, make sure everyone is on the same page. Make sure full consent is practiced by all parties involved, everyone knows what's expected, what the boundaries are (if any), and that they will be respected by everyone. Again, too many horror stories of these situations when one person didn't really want what was offered, allowed it to unfold anyway, and then it ends in disaster with people's feelings (or worse) left in tatters.
Full, open, honest communication from all parties at all times about what they want, what they don't want, what they expect.
Good luck and have fun!