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New paradigms for property control are emerging as semiconductor research expands to ternary materials. II-IV-N₂ compounds, structural analogs of III-Ns, exhibit cation lattice site disorder that can tune the bandgap and other properties. Here, we investigate MgSnN₂, a II-IV-N₂ with two accessible polymorphs and cation disorder. We use combinatorial co-sputtering to form wurtzite MgSnN₂ up to 500 °C and a metastable, secondary rocksalt phase up to 200 °C. Rocksalt MgSnN₂ has a wide, indirect bandgap and large dielectric constant; wurtzite MgSnN₂ has an ordered bandgap of ~2.3 eV which is reduced by cation disorder, confirmed by spectroscopic ellipsometry. We also grow mixed wurtzite/rocksalt MgSnN₂ heteroepitaxially on GaN at 400 °C. Rocksalt MgSnN₂ is promoted by a close lattice match to GaN along the hexagonal close-packed plane. We utilize this close match in ongoing work investigating cation ordering in rocksalt MgSnN₂ using post-growth annealing. This work enables integration of cation ordering and polymorph formation, uniting existing methods of controlling semiconductor properties with emerging tools.