In near-IR spectroscopy, the concentration change in oxy- and deoxyhemoglobin in tissue is calculated from the change in the detected intensity of light at two wavelengths by solving the simultaneous equation based on the modified Lambert-Beer law. The wavelength-independent constant or mean optical path length is usually assigned to the term of partial optical path length in the simultaneous equation. This insufficient optical path length in the calculation causes crosstalk between the concentration change in oxy- and deoxyhemoglobin. We investigate the crosstalk in the dual-wavelength measurement of oxy- and deoxyhemoglobin theoretically by Monte Carlo simulation to discuss the optimal wavelength pair to minimize the crosstalk. The longer wavelength of the dual-wavelength measurement is fixed at 830 nm and the shorter wavelength is varied from 650 to 780 nm. The optimal wavelength range for pairing with 830 nm for the dual-wavelength measurement of oxy- and deoxyhemoglobin is from 690 to 750 nm. The mean optical path length, which can be obtained by time- and phase-resolved measurement, is effective to reduce the crosstalk in the results of dual-wavelength measurement.