If you attached 2 iPhone Airs, you'd have a chonky foldable at 11.2mm getting into territory where people complained with the previous Samsung.

Samsung's current, Fold 7, is 8.9mm closed. Turns out you can actually go thinner than a thin slab like Samsung Edge (5.8mm) or iPhone Air (5.6mm) because you can move components out of one side and put them in the other, including battery. This lets Samsung build a foldable that's 4.2mm when open, meaningfully slimmer than either the Edge or Air.

My guess is Apple's book-style foldable will be ~4mm open and ~8.5mm closed and just edges out Samsung's Fold 8 when they ship next year.