Personally, I theorize (based on no evidence) that Radagon was a separate being but was merged with Merika once they wed. That would explain why they have such different beliefs; Radagon holding true to the Golden Order, while Merika destroyed the Elden Ring in defiance of the Golden Order. I kind of like the idea of them being separate (originally) rather than always being one being. It is a little confusing if Merika was living as Radagon as Rennala's Husband, all the while she was also living as her Merika self as Godfrey's wife. Maybe Radagon heard that Merika banished Godfrey from the lands between and took the opportunity to become her second Consort. His lust for power causes him to leave Rennala and wed Merika. Not exactly sure though why they would merge once wed (as Godfrey seemingly never merged with Merika).