An approach for the online co-phasing of segmented telescopes using dual wavelength digital holography has been proposed. In this approach, two digital interferograms, one fringe pattern for one wavelength, which are acquired by a high speed CCD camera using a Mach-Zehnder point diffraction interferometer, are required. By these two fringes for two different wavelengths, the phase of a synthetic wavelength for the segmented telescope can be extracted using the digital holography technique. To overcome errors caused by atmosphere and measurement noises, the piston and tip/tilt coefficients for each segment of the telescope are acquired by fitting the plane from the phase of the synthetic wavelength in each segment of the telescope. The performance of the method is tested by co-phasing of a simulated telescope with 37 hexagonal segments with the existence of turbulence and noise. The numerical experimental results show that co-phasing of the segmented telescope with high accuracy can be achieved by dual wavelength digital holography.