I don't know if they can be merged, it depends on the intended use of boundaries.
Are boundaries only suppose to link to one circle?
SO can I create one boundary that allows all permissions to say circle named "Family"? Can I then create another boundary that is linked again to the same circle "Family", but then give it more restrictive permissions like the post can't be shared? Then if I want to share a post with "Family" I then have the option of allowing or not allowing them to share the post depending on the boundary I choose to use?