A　phase　retrieval　method　for　dual-wavelength　in-line　digital　holography　is　presented　with　double　axially　displaced　holograms.　A　synthetic　wavelength　is　used　during　iterative　propagations　to　retrieve　wrap-free　phase　distributions　with　a　much　extended　measurement　range.　The　simulation　and　experimental　results　demonstrate　a　better　elimination　of　the　twin　image,　a　faster　rate　of　convergence　of　the　iterative　routine　and　less　number　of　wavelengths　are　compared　with　previously　reported　multiple-wavelength　in-line　holography.