Comment by wtallis

If the outer screen has dimensions X by Y, then you unfold it to have the inner screen with dimensions 2X by Y, which cleanly permits side by side apps at the original dimensions of X by Y. This is true for any ratio between X and Y.

If the user isn't going to be multitasking, then the app needs to additionally support the widescreen dimensions of 2X by Y, and the portrait orientation of Y by 2X. Both of these are so drastically larger overall that aspect ratio is a barely-relevant part of the UI layout problem, because nobody wants every button and text label to be double the size (which would mostly defeat the benefits of having the larger screen for anything other than multitasking).