We present a simple and low-cost approach to generate dual combs from a single fiber-ring cavity based on nonlinear multimode interference. A single-mode fiber-graded-index multimode-single-mode fiber structure serves as an all-fiber saturable absorber for mode-locking. A pair of frequency combs with different comb spacing are generated by dual-wavelength and polarization-multiplexed mechanisms. Stable and simplified dual-comb spectroscopy is achieved using the proposed free-running fiber laser, with no need for complex phase-locking configurations. As a principle of concept, we demonstrate the dual-comb fiber laser for spectral determination of fiber Bragg grating (FBG)-based strain sensors, providing a low-cost alternative for interrogation of FBG-based sensing networks.