Wavelength-division multiplexing is demonstrated for a set of two doubly clamped beams. Using a single input/output waveguide in a nanophotonic detection system, the two mechanical beams are independently addressable using different wavelength channels as determined by their respective racetrack resonator detection cavities. The two cavities slightly overlap, which also enables the mechanical frequency of both beams to be detected simultaneously with a single wavelength. Finally, to physically map which wavelength channel corresponds to which specific device, a heating laser is targeted individually on each beam to create a reversible mechanical frequency shift. This multiplexing method would allow for the simpler detection of large arrays of nanomechanical devices in a sensor system.