We report a refractive index sensor comprising of unique T grating on top of periodic nano-cavities. The sensor has two resonant modes sensitive to different regions of the structure with low inter-region interference, hence allows simultaneous detection of two different analytes or more accurate detection of a single analyte. The sensor also provides a self-referencing feature for a broad range of refractive index, from 1.3 to 1.5. Using the FDTD method, the sensitivities of 801.7 nm RIUtextsuperscript-1 and 1386.8 nm RIUtextsuperscript-1 have been recorded for the two modes respectively. The versatility of the structure makes the sensor a prominent candidate for biochemical and other sensing applications.