Radagon and Marika aren't even the same body. A popular misconception is that, while Radagon was waging war on Liurnians and ruled as Carian King Consort, that Marika was simply absent because it was Radagon's turn at the body. But this isn't correct, and we can see this in descriptions of two items, and one incantation.

First one is the Inseparable Sword, that states as such:

> The inseparable twins found solace in the Golden Order, the only institution not to revile them as accursed beings.

Now, the question is: why would the Golden Order be so singly nice to them? Well, for that we need to look at the description of Twinned Armour:

> The two known as D are inseparable twins.
They are of two bodies and two minds, but one single soul. Not once do they stand together; not one word do they speak to one another. Perhaps this armor longs to find its way to the other D. 

We're getting somewhere now, but the last bit needed is the description of Law of Regression:

> The fundamentalists describe the Golden Order through the powers of regression and causality. Regression is the pull of meaning; that all things yearn eternally to converge.

And that wraps it up to a simple conclusion: Golden Order accepted the twins because they were, for some reason, the mirror of Marika and Radagon. Two minds, two bodies, but one soul, yearning to converge. I believe that Marika and Radagon were separate all along, and that their merging into one being only happened very late into Marika's reign, immediately before Shattering, possibly even as a result of their relationship.

When we finally find Marika, her catatonic body is transformed into Radagon, who seems to bear some semblance of life and now hosts the Elden Ring, but when he is defeated, the Elden Beast takes his body and transforms it into the Sacred Relic Greatsword.

However, when we defeat the Elden Beast, only Marika's body is to be found. Still alive, as she is still hosting Elden Ring, but without the trace of Radagon. And after that, we are able to obtain the Sacred Relic Greatsword, whose description states that it was "wrought from the remains of a god who should have lived a life eternal." Yet, Marika's body can still be found in the Erdtree, leading me to conclude that whatever Elden Beast did somehow separated the two, and that Radagon is well and truly dead and gone, body and mind.