Interactions between poly(sodium styrenesulfonate) (NaPSS) and sodium polyacrylate-acrylamide copolymers, NaP(AA/AM), in aqueous solutions have been studied. Equilibrium pseudoternary phase diagrams have been determined, for varying combinations of molecular weights and charge densities of the components and with varying concentrations of salt in solution. All the fully charged mixtures segregate at sufficiently high concentrations of polyelectrolytes (typically 6-8 wt %). Addition of salt and/or increasing the molecular weight results in a larger two-phase area in the diagrams. The main finding on reducing the charge density of one of the components P(AA/AM) is that the extent of the two-phase region changes relatively little until the charge density is reduced to about 20%, where, abruptly, complete miscibility occurs. The phase behavior is qualitatively reproduced by a modified Flory-Huggins model where dissociated counterions are included as a separate component contributing to the entropy of mixing. Experimental observations and theoretical modeling suggest that the model presented is a promising tool for achieving better quantitative, as well as qualitative, predictions of phase behavior of polyelectrolyte-polyelectrolyte-solvent systems, if appropriate modifications are implemented.