We want someone to be with both of us equally
What she hears: "My feelings don't matter."
You probably want someone to love you both because you feel like you've got a lot of love to give and it would be awesome for everyone to all be in love, right?
Well, in all recorded human history, nobody has ever fallen in love with two different people at the same rate in the same way at the same time. It doesn't happen.
There are many triads out there. Almost all of them started when one member of a couple met someone, they started dating, and then later, that person started developing an attraction for the other member of the couple. When you insist that someone must love both of you equally, you are saying you think she can just wave a wand and magically control her heart. The heart doesn't work like that.
Worse, you're telling her if she doesn't feel the same for both of you--and she won't--she can't be honest about it.
We are a package deal; the relationship is with both of us
What she hears: "If you invest in a relationship with us, spend time and emotional energy with us, and then later down the road you decide you don't want to be with one of us, we will stomp all over your heart."
Coming as a package deal probably sounds like a great way to keep a third person from coming between you and damaging your relationship, right?
You're setting yourselves up to be coercive. Say you find local unicorns, everything's awesome, maybe she even moves in with you. Then, later on, something happens. Who knows what. Maybe the chemistry isn't there. Maybe that little annoying thing she thought she could overlook turned out to be not so little.
Anyway, she doesn't want to be with one of you any more, but she sincerely, truly, deeply loves the other one of you. Now what do you do? If you're like most unicorn hunters, you tell her "sorry, we are all or nothing."
Think about what that does to her. You're telling her "you have a choice: either be intimate with someone you don't want to be intimate with, or lose your relationship with someone you love" (and become homeless, if she moved in with you).
What a yucky, horrible thing to do to someone. People who have good self-esteem, healthy relationship skills, good communication, and a solid sense of self-confidence--in other words, the kind of people you want to dater going to take one look at that arrangement and head for the hills.
It's also a horrible, yucky thing to do to your other partner. Think about what you are saying. "Hey honey, if the person you love doesn't put out for me too, I'm going to order you to break up with her." That is going to create stress in your relationship for sure. Telling someone you love to break up with someone they love is not a good way to build relationship stability.
You will find people who are willing to be with both of you under these conditions as long as it's just sex. But anyone who thinks it through is not going to want to fall in love with you, because a problem with one of you means the other one will break her heart, and that's not cool.