iOS 27.2 brings Apple Pencil support to the iPhone Duo, and only one Pencil qualifies: the Apple Pencil with USB-C. MacRumors bought one to test on the 27.2 beta. Pairing is a cable job, and the Pencil ships without a cable, so plug in the one from the Duo's box, accept the prompt, and it works right away.
A new Apple Pencil page in Settings shows the charge level and adds two switches: Only Draw with Apple Pencil, which leaves fingers for scrolling, and Scribble for turning handwriting into text. Unfolded, the 7.6-inch screen gives notes and markup enough room, but in MacRumors' test the phone rocks on its camera bump, more so toward the left corners, and Apple's kickstand case only dulls it. The cover screen manages a signature and not much more. The Pencil also clings magnetically to the hinge, which MacRumors doesn't think Apple advertises.
The worry going in was the soft inner screen. Writing leaves faint lines and traces of words, and a polishing cloth clears them completely, per the video. The nano-texture also gives a paper feel the video likes, the same as on nano-texture iPads:
I think it does a really good job of kind of feeling like you're writing on paper.
MacRumors sees the clearest win for field workers marking up documents, one device where they used to carry two. Other iPhoners get a full-size Pencil that rides the hinge on a desk but has no place in a pocket, and I'd wait for a case that carries one before buying in.