The screen swap isn’t the same with what i’m talking about though. It just inverts the position of the screens.
Both cores (Desmume and Citra) have other screen options in the core options that remove the bottom screen completely so you can have the main screen fill your entire monitor.
Many games don’t really need the bottom screen all the time, just when you need to change something so it would be nice if you could make it appear or remove it with a hotkey instead of having two small screens all the time or having to go to the core options to manually change it every time. You can do this with the Citra standalone because it gives you the option to bind a hotkey to this function.