Developer Gustave Monce has been working on installing Windows 11 on their own Surface Duo for a while now and now you can try it on your own at your own risk as a guide of sorts is now available (via Neowin.)
Available on Github, this new guide is the result of Gustave's tweaking. It is only for the 128GB Surface Duo (the Duo 2 is not supported,) and is not an easy task, to say the least. As you'd expect, it involves using a custom UEFI and dealing with drivers and other sensitive things. You'll also have to unlock the bootloader, and create custom partitions, too.
— Simone Franco (@Simizfo) March 10, 2022
If all goes well, you can end up enjoying native Windows 11 on your Duo, though some things in Windows like touch, cellular, cameras, might not work right. Fair warning, as another developer puts it, "almost nothing works right now."
So, if you decide to try this at your own risk, you'll pretty much end up with a niche device that can't do much. Still, it's a nice little side project, especially if you're highly technical, and want a peek at what things would have been like if the dual-screen Surface Neo tablet was never canceled.