With people talking about the two Pures eventually crossing paths, I assumed that the time travel here works like The Butterfly Effect where her mind is sent back in time to her body in the past so there is only one Pure at any given moment. No paradox and no self-rivalry.
Yeah but that kind of time travel makes no sense to me - when you are in the past your future self does not exist - so if you overwrite her mind with your future one where does HERS go? You would in effect be killing your past self and replacing it - our brains aren't really only 10% used - all our neurons are doing stuff and there is no room to have her mind stored in there too somewhere.
Also your only option would be to stay back in time because you have no time travel 'device' with you - your mind couldn't be pulled back to the none existent future by any tech there - that doesn't exist yet.
I guess this method would work if you sent your mind into an alternate reality past version of you - but you will still be 'killing' them.
You need to watch the Butterfly Effect then, I'll use spoiler tags if you feel like you want to watch it before reading this. In the movie the main character basically does overwrite his younger self. The movie actually starts out showing that the kid grew up having various black outs where he did things he couldn't remember. The MC has an ability, that he inherited from his father, to be able to send his mind back in time to change the past. He can do it at will and has no problem sending it back to the future to see how he changed the time line. You can also read Ashita, Kimi ni Aetara since the MC seems to have the same ability but is able to send her mind forward in time to see how what she does in the present effects the future, and it basically runs on the same principle that she overwrites her future self's mind until she is sent back to the present. I don't think the idea of "killing" yourself by replacing your mind with another version from a different time is all the unethical. I mean, sending your mind back would just be you with just a lot more experience, basically Homura in a nutshell.
I also don't think Pure even wanted to go back to the future anyways. If it's through either technology or magic I think Pure knew it was a one way trip. The whole point is to spend her high school years with her future sweetheart.
About the whole brain science, multiple personalty disorder is a real thing. I don't think the mind has that much trouble dealing with more than just one person hanging out in there. I believe it's theorized that everyone actually has two minds anyways, where one is dominate and the other is subdued. I'm no where qualified enough to explain it but from what I remember people who get their left and right lobes separated from surgery to treat severe epilepsy can actually have the subdued half express itself. Such as the two lobes can disagree to do something, like one can make the right hand pull up their pants while the left hands starts to pull them down because they can't agree on whether or not they want to get dressed. I believe there was an experiment where a split-brain patient felt an object with their left hand but could not see it with their right eye, they were not able to verbally identify what they were touching but they could use their left hand to write down the correct identity. The brain is an amazing organ, I think it's futile to try to theorize how it would handle a future copy of the mind it currently has coming back to hold up residence.
