There are also internal changes. The Pro Duo 15 now has a new Intel 10th-gen processor (up to Core i9) and Nvidia's new GeForce RTX 3070 GPU. The Duo 14 is getting Intel's 11th gen processor (up to Core i7) and Iris Xe integrated graphics.
Since the secondary screen is larger than Apple's Touch Bar (but still not very big), you'll presumably use it to display something like a scrolling Twitter feed, or maybe Slack chats, while you use the primary one to get shit done.
Ergonomic nightmare — Unfortunately, in practice, it might annoy you more than anything. Looking at the picture above you can see there's nowhere to rest your wrists. Reviewers have said the design is just plain uncomfortable to use. And understandably that second display makes for poor battery life; last year's generation only got maybe three hours of life on a full charge.
In defense of the ZenBook, it might be useful for some specific productivity work like video editing where you could use the secondary display to show a video timeline — Adobe, blessedly, has built native support in its apps to take advantage of the ZenBook Duo's second screen. Programmers could display a code editor on the primary screen and command line logs on the bottom.
Still, you are trading limited screen space for a heavy compromise on ergonomics — creating a completely new problem in need of a solution. Maybe that's fine with you if the screen space is more important, but for the rest of us, we'll stick to single-display laptops, thank you very much.