Author clearly did their research on how DID works IRL, going by the medical info dump in previous chaps, which says that DID is still the same person, but with a mental disorder.
But then they went and used the fictional fantasy version of DID which falsely says it is "two different people in 1 body".
If they wanted to do the fantasy version of DID anyway, then don't go wasting our time making us think this was the real version of DID.