Online co-phasing of segmented telescopes by dual-wavelength digital holography is proposed.Two digital holograms (one hologram per wavelength) are employed to get the phase of the segmented telescope for a longer synthetic wavelength by the digital holographic approach. The holograms are recorded by a high-speed CCD camera using the point-diffraction Mach-Zehnder interferometer. By fitting the plane for the synthetic wavelength phase in each segment of the telescope, the coefficients of piston and tip/tilt for each segment can be acquired. To test the feasibility of the proposed method, two simulated telescopes with 37 hexagonal segments are taken as examples to examine co-phasing segmented telescopes by digital holography when atmospheric turbulence and detection noises exist. Numerical simulations show that the dual-wavelength digital holographic approach for co-phasing of segmented telescopes is feasible and highly accurate and robust.