Apple executives John Ternus, Molly Anderson, and Steve Lemay discussed the iPhone Duo's design inspiration with GQ magazine, citing ordinary paper notebooks as a key influence. Ternus referenced Steve Jobs' vision of the intersection of technology and liberal arts, calling the iPhone Duo a perfect example. Anderson, Apple's vice president of industrial design, explained that the goal was to recreate the familiar feel of a notebook in the pocket while achieving compactness and usability.

The device's proportions draw from the international A-series paper system, closest to A5, though Apple describes it as passport-sized when closed. Lemay, vice president of human interface design, emphasized seamless software integration, aiming to avoid the feeling of switching operating systems when unfolding the device. The hinge features a custom variable torque profile for smooth opening and stable flat positioning.

The team focused on emulating the convenience of paper notebooks while creating something entirely new, blending learnings from iPad, Macintosh, and existing iPhones.