Pretty sure Naka never viewed the games as taking place in two worlds. There was that interview from the early 2010s in which he was asked where all the humans that started appearing from SA1 onward came from and he explained that the old games took place in islands, and the story later moved to places like Station Square. No dual world mention. Edit - found the interview: "Sonic initially had critters and robots but Sonic Adventure have introduced humans suddenly. Why have you decided to add them? Naka: Action in Sonic's universe unfolded on Christmas Island from the very beginning, and accidentally things have happened the way there were only animals and robots in there. And later when Sonic made it to usual mega polis in Adventure, that's where he met humans."