Spinoidal and critical points of the liquid-liquid equilibrium in random copolymer blends are calculated by application of continuous thermodynamics. Blends are composed of two copolymers, each consisting of two different monomer units. Both copolymers are characterized by divariate distribution functions with respect to molecular weight and to chemical composition. Based on a model for the excess Gibbs free energy of mixing, the necessary relations are derived for the calculation of spinoidal and critical points. The influences of the polydispersities of molecular weight and/or chemical composition on the instability limit are discussed. Miscibility maps are calculated on the basis of the critical point. Cloud-point curves are shown to be most sensitive to polydispersities if the excess Gibbs free energy of mixing exceeds only slightly the lowest value necessary for demixing.