We report the first experimental demonstration of a heterodyne self-oscillator operating simultaneously on the first and second mode of a silicon NEMS resonator. This architecture tackles the simultaneous monitoring of a doubly-clamped resonator's first two resonant modes required for real world applications in the field of biological mass sensing. This oscillator is based on a downmixing scheme where the NEMS motion-induced electrical signal around 25 MHz and 70 MHz is shifted down to few tens of kilohertz thus reducing the bandwidth constraint on electronics and limiting the feedthrough between the NEMS actuation and detection electrode. In this paper, we present the oscillator measurement results in open-loop and closed-loop, and we evaluate the frequency stability. When implemented on silicon this oscillator scheme will help to circumvent today's limited NEMS integration density mostly due to a bulky circuitry. © 2016 IEEE.