I'm note sure if Emiline's state is completely intentional. From what her mother told us, the fusion maybe happened when she touched the dragon's egg. Julian might be crazy, but he pays attention, and he's not stupid. He saw that his daughter was a potential superweapon and capitalized on it. Well, her plus the staff.
This also makes me wonder if this is actually Ermiline's "true" form. Her reyn form might have been one of convenience, the stillborn kit was just a coincidence... Or there could have been three entities wrapped up in that body as well. Two parts dragon light and dark, and one part reyn...