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Dual-salt magnesium/lithium-ion batteries (MLIBs) benefit from fast lithium ion diffusion on the cathode side while providing safety due to the dendrite-free Mg²⁺ stripping/plating mechanism on the anode side. Bulk MoS₂ (B-MoS₂ ), as a cathode for magnesium-ion batteries (MIBs), suffers from low conductivity and relatively van der Waals gaps and, consequently, resists against divalent Mg²⁺ insertion due to the high Coulombic interactions. In MLIBs, it exhibits a Daniell-cell type mechanism with the sole accommodation of Li⁺ . In this paper, the synthesis of a 1T/2H mixed-phase MoS₂ (MP-MoS₂ ) modified with a hyperbranched polyethylene ionomer, I@MP-MoS₂ , for high-capacity MLIBs with a distinct Mg²⁺ /Li⁺ co-intercalation mechanism is reported. Benefiting from the enhanced conductivity (due to 53% metallic 1T phase), expanded van der Waals gaps (79% expansion compared to B-MoS₂ , 1.11 vs 0.62 nm), and enhanced interactions with THF-based electrolytes following the modification, I@MP-MoS₂ shows a dramatically increased Mg²⁺ storage compared to its parent analogue (144 mAh g^-1 vs ˜2 mAh g^-1 at 20 mA g^-1 ). In MLIBs, I@MP-MoS₂ is demonstrated to exhibit remarkable specific capacities up to ˜270 mAh g^-1 at 20 mA g^-1 through a Mg²⁺ /Li⁺ co-intercalation mechanism with 87% of capacity retention over 200 cycles at 100 mA g^-1 .