Component order parameters and phase fields corresponding to nematic liquid crystals of various symmetries were determined for a nematic mixture of biaxial rod-like and plate-like molecules (parallelepipeds of D_2h symmetry), interaction between which is pure steric repulsion. The problems of stability of the biaxial phase are discussed. The region of this phase is shown to be asymmetric with respect to the equimolecuar composition, in agreement with numerical experiment results.