Mg2+ secondary batteries are remarkably safe, resourceful, and exhibit high energy density. However, the excessively slow reaction kinetics at Mg2+-battery cathode materials results in charge–discharge over 10–60 h at room temperature, hindering the performance evaluation and mechanistic analysis of the electrode materials. In this study, we developed a dual-salt electrolyte comprising a conventional magnesium salt, magnesium bis(trifluoromethanesulfonyl)imide [Mg(TFSA)2], and a quaternary ammonium salt, spiro-(1,1′)-bipyrolidinium tetrafluoroborate (SBPBF4), for achieving high-rate performance in the cathode reaction. In a charge–discharge test conducted using a highly defective FePO4 cathode, the dual-salt system [0.5 M Mg(TFSA)2 + 0.5–2.0 M SBPBF4] showed a high capacity of over 150 mAh g–1 at 0.5C-rate, even at room temperature. In situ X-ray absorption fine structure measurements demonstrated the Fe2+/Fe3+ redox reaction of the FePO4 cathode during the charge–discharge, whereas Raman analysis and molecular dynamics simulation indicated that the multiple-anion-coordinated [Mg2+–BF4 –] structure was more effective in facilitating Mg2+ insertion/extraction than the [Mg2+–TFSA–] structure, which has a lower number of coordinated anions. These findings indicate that the Mg2+ insertion/extraction at the cathode/electrolyte interface is drastically improved by using a combination of typically used electrolytic salts as the electrolyte. This strategy enables rapid evaluation of the electrochemical performance of various Mg2+-battery cathodes without high-temperature and prolonged operation.