A phenomenological model for predicting phase diagrams of a binary nematic mixture containing side chain liquid crystalline polymers and/or low molar mass liquid crystals has been proposed by combining Flory-Huggins free energy of isotropic mixing and Maier-Saupe free energy for nematic ordering of the nematogens. Two orientational order parameters, s(1) and S-2, Of the two components in the mixtures having two different clearing temperatures are taken into consideration in the calculation. The Flory-Huggins interaction parameter, chi, and the nematic interaction parameter of the Maier-Saupe theory, nu(1) and nu(22), are assumed to be functions of inverse absolute temperature. Further the cross-nematic interaction is assumed to be proportional to the square root of the product of the nematic interaction parameters of the two mesogens, i.e., nu(12)=c (nu(11).nu(22))(1/2). The theory predicts a variety of phase diagrams depending on a single parameter, c, which is a measure of a relative strength of interaction between two dissimilar mesogens to that in the same species. The predicted phase diagrams have been tested rigorously with experimental phase diagrams of various nematic mixtures reported by others as well as by us.