The strategy of alloying in the Mg3Sb2-Mg3Bi2 thermoelectric compound has originally been understood mostly as a means to reducing the thermal conductivity. However, it is evident from electronic transport properties that alloying also has a significant impact on the band structure. To fully understand the optimum alloy composition for thermoelectrics, it is necessary to model both the p- and n-type compounds as a function of the Mg3Sb2 vs. Mg3Bi2 composition. We establish a model for the optimum alloy composition and find that the electronic property enhancement accounts for about 50 % of the benefits from alloying. We discuss how the Mg3Sb2-Mg3Bi2 alloying impacts the band structure in terms of the band gap, mass, and convergence, which are the essential features that should be considered for band engineering in this material.