Oh wow.
A very interesting hook in the isekai genre. What the hell? I can't really think of one off the top of my head whose made use of this as the core premise of the story either. I know a few that have dual personalities, sealed up personalities, the concept of replaced personalities which is usually done, or fused personalities. I think the closest I can think of where clearly the two personalities are going to be antagonistic to one another was the one with the reincarnated into a dead hero's body. I forget the name of that one...
There's already some very solid worldbuilding right off the bat. A lot of tragedy, a lot of potential deep meaningful drama. And a subversion too, making you think we'll be following the typical isekai protagonist but moving instead to the person within the fantasy world who was replaced.
Now to see if we get a dual protagonist POV or if we're mainly going to follow one or the other. And both are victims of circumstances too, although one is given a new chance at life at the expanse of the existing life, so the one in the fantasy world has clearly been given the more dreadful set of circumstances to overcome.