I take it the kind of relationship you're looking for is yourself (a hetero) male, Amanda, and one other bisexual female with whom you and Amanda both have a romantic relationship. To wit, a triad, as any three-person unit with all three persons romantically involved with each other is called a triad.
The "problem" is that you're looking for a "unicorn triad." By that I mean, two bisexual females, and one heterosexual male, with one of the females being a "third," and the other being the man's primary partner. The man and woman who were together in the first place are the original couple and as such, get what we sometimes call "couple privelage." That means the woman who is a "newcomer" to the scene kind of gets second-class status compared to the original couple. She doesn't get to make the rules; she doesn't get a say in discussions about the rules. She just has to accept whatever the "primary couple" tells her.
From there, other problems crop up. The "third" -- the "unicorn" -- is often expected to be equally in love with both members of the original couple, and is "forbidden" to fall in love with anyone else outside the triad. Sometimes this "unicorn woman" is expected to do the chores, take care of the kids, etc. ... She is archtypically financially dependent on the original couple, which cements her obligations to them. Also, she might be treated like a "dirty little secret" who is not to be revealed to any of the original couple's family members, nor invited to any of their family functions. Stuff like that.
Obviously, such a set-up is doomed for disaster; it's just a matter of when. Definitely not saying this is how you and Amanda would treat your "third," but you'd be surprised how many couples do treat their third that way. What I am saying is, take precautions so that you don't inadvertently become "unicorn hunters." Have a good deal of flexibility toward whatever woman you meet. Maybe she'll only fall in love with one of you. Maybe she'll already have a (polyamorous) partner. Maybe she'll already have kids (with all the responsibility they entail). And so on. Just don't saddle her with expectations that she'll be the "perfect woman for you." The perfect woman doesn't exist; that's why we call her a unicorn.
Having said that: MFF triads do occur, and can certainly be successful and fair to all three persons (especially if all three get a fair and equal say in the family discussions and any rules that may result). So sure, be optimistic that a nicely-balanced MFF triad may be in your future. But be open-minded about the other possibilities as well. Just as Amanda's need for another woman was unexpected, so the solution to the problem may also take an entirely unexpected form. Which is okay, because it might actually be better than you had imagined.
Love means never having to say, "Put down that meat cleaver!"