Polyoxometalates (POMs) are ideal components for reversible multi‐electron storage in energy technologies. To‐date, most redox‐applications employ only single, individual POM species, which limits the number of electrons that can be stored within a given potential window. Here, a synthetic approach is reported, where spontaneous redox self‐equilibration leads to the formation of two structurally related polyoxovanadates which subsequently aggregate into co‐crystals. This results in systems with significantly increased redox reactivity. The mixed POM system was formed by non‐aqueous self‐assembly of a vanadate precursor in the presence of Mg2+, resulting in two mixed‐valent (VIV/V) species, [(MgOH)V13O33Cl]4‐ (= {MgV13}) and the di‐vanadium‐functionalized species [V14O34Cl]4‐ (= {V14}), which co‐crystallize in a 1:1 molar stoichiometry. Experimental data indicate that in the native state, {MgV13} is reduced by three electrons, and {V14} is reduced by five electrons. Electrochemical studies in solution show, that the system can reversibly undergo up to fourteen redox transitions (tentatively assigned to twelve 1‐electron processes and two 2‐electron processes) in the potential range between ‐2.15 V to +1.35 V (vs Fc+/Fc). The study demonstrates how highly redox‐active, well‐defined molecular mixtures of mixed‐valent molecular metal oxides can be accessed by redox‐equilibration during synthesis, opening new avenues for molecular energy storage.