A method for dual harmonization of a DDP posture-detecting subsystem integrated into a worn/borne head-up viewing system includes a first step of measurements of N rotation matrices {circumflex over (K)}l for detecting the posture of a head corresponding to a set of different targeting actions Vi, in which measurements one or more different preset elements of pilot/driver information displayed in the viewing device Dv are superposed or aligned with one or more corresponding landmarks of the real outside world; then a second step of conjointly calculating the relative orientation matrix {circumflex over (R)}(S1/v) of the DDP tracking first element S1 with respect to the viewing device Dv and/or the relative orientation matrix {circumflex over (R)}(ref/S2) of the external reference device DRef with respect to the DDP fixed solid second element S2 to respectively be the right-side bias rotation matrix {circumflex over (D)} and the left-side bias rotation matrix Ĝ, which are solutions of the system of dual harmonization equations: Ûi=Ĝ·{circumflex over (K)}i·{circumflex over (D)}, i varying from 1 to N.