In our previous work, we proposed an expression for the Helmholtz energy using universal parameters correlated with energy-of-mixing results from Monte Carlo simulation for binary polymer-solvent solutions. In this study, we obtain new universal parameters for polymer/polymer alloy systems from Monte Carlo simulation data and we extend our previous model to polymer alloy systems. In addition, we introduce a specific interaction term based on a modified double-lattice model. The model includes the secondary lattice as a perturbation term to account for specific interactions. The coexistence curves generated by the proposed model are compared with experimental data.