A short-range Sutherland potential is mapped with the two-Yukawa potential and incorporated into the first-order mean spherical approximation (FMSA) theory to deal with the short-range dispersion interactions of ion-ion, ion-solvent, and solvent-solvent. An equation of state (EOS) based on primitive MSA and FMSA is constructed to describe the single- and multiple-salt solutions. With the universal and transferable ionic parameters derived from mean ionic activity coefficients and solution densities of single-salt solutions for five cations (Li+, Na+, K+, Ca2+, Mg2+) and five anions (Cl-, Br-, I-, NO3-, SO42- ), the proposed EOS predicts the correct osmotic coefficients as well as water activities for 19 monovalent and bivalent two-salt solutions. Without any additional mixing parameter, the predicted osmotic coefficients for aqueous two-salt solutions are in good agreement with experimental data.