It is not 2 games at the same time. LOL
It's effectively one game and when entering the spiritual world, it's the same scene but with different assets, lighting, and other underlying graphical effects. 2 different versions of the game at the same time would be a heavy task even for 3090. Maybe impossible.
Running 2 games at the same time would be rendering 2x the full screen resolution, but that's not exactly the case here since the 2 viewports share the same screen estate.
So in this case the rendered number of pixels would be the same regardless if it's 1 or 2 scenes.
(for example 2x4k vs 4K split in 2 scenes).
Core of the scene is the same on both shared screen, same building structure and environment, but with different assets, lighting, and other underlying graphical effects. And when that appears, everything drops in resolution to 900p and texture quality. And of course it's 30fps
Yeah, scenes and 2 different games is the same. LOL
About that, you should ask yourself, not others