In this paper, a simple approach to simultaneous spectrophotometric determination of phosphate and silicate, based on measurements of parameters of a single peak and two-component calibration, is presented. A two- stream flow injection system was developed for the method implementation and the absorbance measured for the mixture of molybdophosphoric and molybdosilicic acids was applied to broaden the calibration range for phosphate determination. As it was observed that the presence of silicate influenced the analytical signal of phosphate, the calibration model based on preparation of four two-component standard solutions of analyte concentrations corresponding to a 22 factorial design was used. The approach was verified using synthetic and certified reference material samples of groundwater, wastewater, and river water.