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).

But, the iPhone Duo has a different aspect ratio from a regular iPhone. So your app needs to be designed for multiple aspect ratios anyway.

The inside screen, when divided in two vertically, along the fold line, is exactly the same dimensions as the outer screen. Pretty much by definition. Any folding phone, of any aspect ratio, can run two apps on the interior display side by side.

The issue of the internal screen being the same aspect ratio as the external when rotated is completely separate.

which they have to be anyway, to support normal iphones and maybe ipads and in many cases a cackle of Android phones too. (but I still think this is brilliant design)