Lauren,
I think I understand. I was assuming that you have a parent-child relationship between the object in Field A and the object in Field B. I think what you are saying is that you don’t have any relationship between the objects and that you would need more than just a parent-child. You need a many to many relationship between object A and B.
I think your best option is to setup a junction object between A and B. Then in your page, setup a model action that runs when Field A is changed. The action would set a condition on a model on the Junction object for field A, query the model, and bring back a list of object B that you would show in your reference field.
Thanks,
Bill
