The iPhone Duo likely does not support the Apple Pencil Pro due to the absence of a magnetic charging surface. The Apple Pencil Pro requires magnetic attachment for both charging and pairing with compatible iPads. Without this magnetic support, the Duo cannot even establish a connection with the Pencil Pro. The Apple Pencil with USB-C, however, charges through a hidden USB-C port and is the only model currently compatible with the Duo. The general rule is that if a device can physically connect or magnetically attach to a Pencil, it can use it. Devices with both magnetic attachment and a USB-C port can use either the Pencil Pro or the regular Pencil. Conversely, devices lacking magnetic attachment are limited to the regular Pencil. This fragmented compatibility forces users with multiple Apple devices to carry different Pencils for each. For example, an iPad Pro user with a Pencil Pro would need to purchase and carry a separate USB-C Pencil for their iPhone Duo. This situation highlights the ongoing complexity and inconvenience of Apple’s Pencil compatibility across its product line.