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.