We have conducted extensive theoretical and experimental investigations to unravel the origin of the electrochemical properties of hybrid Mg2+/Li+ rechargeable batteries at the atomistic and macroscopic levels. By revealing the thermodynamics of Mg2+ and Li+ co-insertion into the Mo6S8 cathode host using density functional theory calculations, we show that there is a threshold Li+ activity for the pristine Mo6S8 cathode to prefer lithiation instead of magnesiation. By precisely controlling the insertion chemistry using a dual-salt electrolyte, we have enabled ultrafast discharge of our battery by achieving 93.6% capacity retention at 20 C and 87.5% at 30 C, respectively, at room temperature.