Were they always two parts of one entity? This line of dialogue doesnt make that any clearer: "O Radagon, leal hound of the Golden Order. Thou'rt yet to become me. Thou'rt yet to become a god. Let us be shattered, both. Mine other self." Radagon was not yet Marika, nor that he was shattered. Did she fuse with Radagon to effectively seal him away to prevent him from claiming the Greater Runes? Radagon was as she implies a stern loyalist of Golden Order, and an incredibly powerful champion while she started to doubt the Golden Order. He would effectively foil Marika's plans left to his own devices.

She only shattered the Elden Ring AFTER merging with Radagon, and he was condemned to the same punishment because he was bound to her.

In the boss fight, Radagon's remains are subsumed by the Elden Beast upon his defeat, while fractured Marika remains in the same pose as she was imprisoned in.
This could show that they no longer shared the same body, as I think the Elden Beast used Radagon's body to forge the Sacred Relic sword.

This is all just wild speculation on my end, but I feel there's definitely something more to it than just "Radagon is Marika"

It was a wonderful day in leyndell, it soon turned nightfall and you could see the wondrous erdtree illuminating the streets, until, godwyn was assassinated by black knifes, which were impossible to detect because of their invisibility, and many others followed, so marika was in grief, like every normal other person would be, until she decided its all the greater wills fault and broke the elden ring, and ruined everybody's life in the whole lands between, instead of trying to capture these black knifes and execute them righteously. What a *****

C'mon, FromSoftware. We need an updated autopsy report.

Marika's gonna definitely win the "Worst Mom in the World" reward, without question.

I think her shattering the elden ring was because she was thinkin "how could the greater will allow this to happen to my boy?" after the night of the black knives killed godwyn the golden

i can understand if someone has more than one personality, but more than one body but still one? that's f*cked up sh1t.
male half, female half, each has their own life, the male had s3x with a magic queen and got some kids, the other had s3x with a lord got some kids too, then back together again and have s3x again and got some new other kids, but after all of that, still considered as one like a dang hermaphrodite but also two bodies at the same time.
i'll just play this for the genre.

Honestly wish i could murk her stupid ***, for all the horrible **** she's done and how much she ****ed everything/everyone up for her own selfish ambitions,Gwyn did what he did out of fear and paranoia, this evil hoe though? Yeah she can absolutely eat my ***, what a **** god to have as a ruler.

Marika was also known as Marika, the 'Eternal'. Black Knives are rumoured to be Numens, who are said to have close ties with Marika. Rogier stated that there are rumours that the Black Knives are scions of 'Eternal' City. Hm, what could be the relation here? Also, seeing as Marika can change shape into Radagon, and her body seems like it's made of some kind of metal, wouldn't that mean she is something similar to the Mimic Tear? If the mimic tears are the result of an attempt to forge a lord, then Radagon was that lord.

It's pretty safe to assume that, at the very least, she shattered the Elden Ring to cancel the authority The Greater Will has over the Lands Between, and she did succeed around the end of the game, seeing how the Two Fingers are losing connection with the Greater Will. Elden Beast, the incarnation of the Order, is the manifestation of the System placed by Greater Will. It imprisoned Marika and fought us simply to protect itself like any system with self-defense protocol would do.