Because this is about two repos with unrelated histories
You can have multiple orphan/root commits, it can be that simple conceptually.
Without reading everything it seems like you can change the paths and still keep the history (of both), while seeing both simultaneously, as opposed to switching branches, like it seems you are saying.
no, I am not talking about switching branches. I mean pushing to a remote repository. Git was designed to be decentralized such that you can push and pull from multiple machines in any flow you like. Most folks at engineering shops use git in a central model from my experience