SPOILER:
Radagon and Marika have one shared body because they were put in a jar ritual and successfuly became-- a saint, with Radagon in control.
Messmer and Melina are children of both Radagon and Gloam Eyed Queen, He made her pregnant before Elden Beast arrived in Lands Between and they were born after Gloam Eyed Queen became an empyrean.
Marika replaced Radagon as the primary persona of their shared body when she was chosen as an empyrean.
Marika adopted the babies and made them her own when she unalived the Gloam Eyed Queen(well technically they are hers(Radagon's).)
Once Marika collected all other empyrean's Elden Ring Rune Copies, She established the Golden Order, using the combined elden ring rune with the rune of death stripped off.
Marika's spirit then left her and Radagon's shared body and embodied the Elden Ring rune, resurrecting inside the erdtree with a Goddess body.
Radagon wakes up with lost memories, became Marika's and the Golden Order's Champion.
Radagon fell in love with Rennala, got married. had happy life with three kids.
The reason why Radagon abruptly went to become Marika's husband is because Marika made him remember and probably threatened him using Messmer and Melina.
Messmer and Melina are children of both Radagon and Gloam Eyed Queen, He made her pregnant before Elden Beast arrived in Lands Between and they were born after Gloam Eyed Queen became an empyrean.
Marika replaced Radagon as the primary persona of their shared body when she was chosen as an empyrean.
Marika adopted the babies and made them her own when she unalived the Gloam Eyed Queen(well technically they are hers(Radagon's).)
Once Marika collected all other empyrean's Elden Ring Rune Copies, She established the Golden Order, using the combined elden ring rune with the rune of death stripped off.
Marika's spirit then left her and Radagon's shared body and embodied the Elden Ring rune, resurrecting inside the erdtree with a Goddess body.
Radagon wakes up with lost memories, became Marika's and the Golden Order's Champion.
Radagon fell in love with Rennala, got married. had happy life with three kids.
The reason why Radagon abruptly went to become Marika's husband is because Marika made him remember and probably threatened him using Messmer and Melina.
Of course this is merely my theory. I hope this game gets another DLC exploring Gloam Eyed Queen.