Cocrystallization of 7,7',8,8'-tetracyanoquinodimethane radical anion (TCNQ-•) and 3-methylpyridinium-1,2,3,5-dithiadiazolyl radical cation (3-MepyDTDA+•) afforded isostructural acetonitrile (MeCN) or propionitrile (EtCN) solvates containing cofacial π dimers of homologous components. Loss of lattice solvent from the diamagnetic solvates above 366 K affords a high-temperature paramagnetic phase containing discrete TCNQ-• and weakly bound π dimers of 3-MepyDTDA+•, as evidenced by X-ray diffraction methods and magnetic susceptibility measurements. Below 268 K, a first-order phase transition occurs, leading to a low-temperature diamagnetic phase with TCNQ-• σ dimer and π dimers of 3-MepyDTDA+•. This study reveals the first example of cooperative interactions between two different organic radical ions leading to magnetic bistability, and these results are central to the future design of multicomponent functional molecular materials.