It appears the iPhone Duo's vertical Dock can currently only house four apps maximum. In his hands-on review, Marques Brownlee attempted to add a fifth app to the Dock but was unable to do so. Apple does not mention this four-app limit in its tech documentation, though those hoping to add a fifth or sixth app may be out of luck.

For the iPhone Duo, Apple made the Dock vertical along the right of the screen, with unused space at either end of it. It will likely take some getting used to for iPhone users, especially left-handed ones. We don't know if Apple will provide a left-handed mode for the rest of us.

Apple today announced the "iPhone Duo," a new kind of iPhone featuring two screens and a foldable inner display. The two halves of the iPhone Duo connect into a single continuous frame with the same aspect ratio across both displays, and Apple said the transition between open and closed is designed to feel fluid. When fully open, Apple says it is the thinnest iPhone ever made.