That confused me too. They were both the same person it was just Sabrina once again wanting her cake (being queen and ruling hell) and eating it too (being a teenage cheerleader hanging with her friends), so she split herself in half so she could just do both. Hell, the Mandrake was more of a "this Sabrina is a different person" than Spellman and Morningstar. Which is why, when the Mandrake Sabrina died I felt something, when Spellman and Morningstar were deciding who should die I really couldn't have cared less. I do think "rock, paper, scissors" was just about the level of decision they needed, since it really was "will I stay and rule hell or will I go back to Earth and hang out with my friends".
I would have been fine with most of the show ending with the "regular" cosmos ceasing to exist as Salem's Endless cosmos took over as the "real" world.