An increased electrification of society calls for a revolution of battery technologies to further improve energy densities, safety and reduce dependencies on critical raw materials. Here we present a new type of fast magnesium electrolytes for all solid-state batteries created as solid solutions of two other fast Mg²⁺ ionic conductors, Mg(BH₄)₂ ∙ NH₃ and Mg(BH₄)₂ ∙ CH₃NH₂. However, the different ligands introduce stacking faults in the structures of the solid solutions, which are eliminated upon heating to T > 40 °C. The stacking faults appear to influence ionic conductivity, as the samples are less conductive after heating. Interestingly, the ionic conductivity does not correlate directly with the relative ligand content, as the highest conductivity is observed for the 1:1 molar composition (σ(Mg²⁺) = 7.3 ∙ 10⁻⁶ S cm⁻¹ at 40 °C), which also has the lowest melting point of 60 °C. Thus, this work demonstrates a new approach to increase cationic conductivity using mixed ligand systems to alter conduction pathways and introduce microstructural strain.