The phenomenon of dual resonance coupling exists in few-mode long period grating coupled between core modes LP01 and LP11. The two resonant dips drift to opposite bands as temperature or strain increases. We research the mechanism of dual resonance from theory and experiment, and design a dual-parameter sensor of temperature and strain. Compared with the long period gratings coupled between core fundamental mode and cladding mode, this sensor has the advantages of refractive index insensitivity, spectrum stability, and high sensitivity.