A molecular thermodynamic model has been proposed that extends the previously known mean field theory of self-association to systems of cross-associating polymers. The extended theory has been applied to model gelation and phase behavior for mixtures of two associating polymers in a good solvent. The gelation threshold is described analytically. Relation between the new theory and SAFT (Macromolecules 2012, 45, 6686-6696 has been established. For ternary systems containing two associating polymers in athermal solvent, simple analytical formula has been obtained for the location of the upper critical consolute point in the long chain limit. Possible types of phase behavior (sol-sol, sol-gel, gel-gel) have been discussed.