<p>In this paper, an analytical formula for the sensitivity of optical sensors based on non-magnetic one-dimensional (1D) binary photonic crystals (PCs) with two defect layers without absorption or gain is derived for the first time. Based on this formula, a comparative analysis of several 1D PC structures with two defects with different types of symmetry is carried out and the fundamental advantage of such PCs over PCs with a single defect is shown. The advantages of this formula over the direct calculation of sensitivity by the shift of the defect mode wavelength at small changes in the refractive index of the defect are shown. Exceptional degeneracy points in 1D PCs with two defects are theoretically obtained. A strong increase in the sensitivity of the defect modes in this case is also shown for the first time. In addition, the exact values of sensitivity for several limiting cases are obtained. A negative sensitivity is demonstrated in specific structures. The theoretical explanation for the possibility of obtaining infinite sensitivity degenerate defect mode in the structure of 1D PC with two defects is presented for the first time. The results of this work allow to considerably simplify the analysis and optimization of optical sensors based on defective photonic nanostructures with two defects. In addition, this work opens perspectives for further investigation of the advantages of PCs with two or more defect layers for the design of optical sensors, filters, and other devices.</p>