A theory of the van der Waals form is used to establish solid -fluid phase diagrams for aqueous solutions containing two kinds of globular proteins. Theory is based on a one-fluid in odd that uses a composition-dependent hard-sphere diameter and a composition-dependent solvent-mediated effective temperature to represent protein solutions. The hard-sphere reference equation of state is based on the model of Young that correlates computer simulations of fluid - solid coexistence curves for binary hard-sphere mixtures. The attractive perturbation term rises the inverse-power potential with variable exponent n. Based on the correlation of George and Wilson and Rosenbaum et al., the optimum range of effective temperature is determined for protein crystallization. For protein systems, as the size difference increases, the mutual solubility of unlike proteins declines sharply in the precipitated solid phase. The theoretical results obtained here may be useful for design of protein-separation processes by crystallization.