The densities of aqueous solutions in the systems CaCl2-NaCl-H2O and MgCl2-NaCl-H2O were determined experimentally at temperatures from (298.15 to 523.15) K, pressures up to 70 MPa and over a range of composition at ionic strengths from (0.1 to 18) mol.kg(-1). The vibrating-tube densimeters used for the experimental measurements have an accuracy on density better than 9.9-10(-5) g.cm(-3). The mean apparent molar volumes of the mixtures calculated from the experimental data permitted a parametrization of the Pitzer equation for mixed electrolyte solutions within the entire range of temperatures, pressures, and compositions covered by this study. The parametrization of the Pitzer model for the pure binary salt aqueous solutions CaCl2-H2O and MgCl2-H2O was also refined based on new experimental and literature data. These models were used to evaluate the partial molar volumes and limiting partial molar volumes of components in aqueous mixtures of electrolytes. Using the refined models for the binary systems, Young's mixing rule can also be used for evaluation of the mean apparent volume of electrolyte mixtures with accuracy similar to the Pitzer model and, consequently, for calculation of the densities of mixed salt solutions from the properties of binary electrolyte solutions. As an important application, we could show that the pressure effect on the estimates of the activity coefficients of components in complex aqueous electrolyte solutions can now be evaluated. Therefore, shifts in equilibrium calculations due to pressure can quantitatively be addressed when modeling geothermal, hydrothermal, or other natural or engineering aqueous systems at elevated temperatures and pressures.