Using a theoretical model, we compare the relative effectiveness of single-component compatibilizers and binary copolymer mixtures in lowering the interfacial tension (gamma) between immiscible homopolymers. The most dramatic effects are seen for mixtures of diblock and comb copolymers, where the mixture can reduce the interfacial tension to zero, even though neither component by itself can produce this effect. Combinations of different comb copolymers are also effective in reducing gamma to values lower than those achieved by either component alone. In the case of symmetric diblocks, the mixture is more effective at lowering gamma than the individual components if the molecular weights of the two copolymers are not significantly different. In general, we find that polydispersity has a beneficial effect on the compatibilizing activity of the diblocks. The results provide guidelines for designing the optimal additives for reducing interfacial tension.