A dual harmonization method for a worn head-up display system for making the display of piloting information of an aircraft on a display conform with the outside real world includes a step of acquisition of N measurements {circumflex over (K)}ι of head posture detection by a series of sightings Vi corresponding to images oriented according to orientations Oi of one and the same photograph taken by a camera, secured to and aligned with the attitude inertial device D3, then a step of joint computation of the matrices of relative orientation M01 and M23 as joint solutions of the dual harmonization Ûi=Ĝ·{circumflex over (K)}i·{circumflex over (D)}, i varying from 1 to N, in which Ûι is the transpose of the matrix M(image(i)/0) of relative orientation of the image(i) of rank i relative to the display D0, with M01 and M23 respectively equal to the right and left matrices {circumflex over (D)} and Ĝ. A worn head-up display system is configured to implement the dual harmonization method.