A dual-wavelength diode laser with sub-MHz narrowband emission at 785 nm is presented. The device is investigated for both emission lines up to an optical power of 150 mW. A stable spectral distance between the two laser wavelengths of 0.6 nm (10 cm-1) over the whole power range is achieved. At 20 mW the emission shows a minimum 3 dB width of 250 kHz and below 1 MHz for output powers up to 80 mW. The results demonstrate that besides the already demonstrated suitability of these devices for Raman spectroscopy and shifted excitation Raman difference spectroscopy, the dual-wavelength diode laser have also the potential for sub-MHz spectroscopic applications.