Aside from the fact they are very much at odds over if they want the Elden Ring shattered or repaired is just about the only signs of two separate minds. Like, if that's the case, why even change gender and hair if it's a shared body? Why be one entity? We learn Merika came from the Shadow Lands but I'm not even sure we ever learn where the hell Radagon came from. Without that piece of the puzzle he really does seem to be of Merika's making. I don't know why though.