The "Iso-W" approach recently proposed to generate simple flow rules for two phase materials is extended to nonlinear viscoplastic constituents and generalized, which produces three families of models. Comparison with the Hashin-Shtrikman bounds leads to select the variant assuming strain rate compatibility and stress equilibrium. The Iso-W assumption is in fairly well agreement with the Mori-Tanaka and self-consistent models for mixtures of both linear and nonlinear materials.