Dual-ion batteries are being considered a feasible approach for electrochemical energy storage. In this battery technology both cations and anions are involved in the redox reactions, respectively, at the anode and the cathode. However, the participation of both ions in the redox reactions means that enough salt must be added in the electrolyte to ensure proper battery functioning, which present a limiting factor in battery design. Herein, a modified version of the standard pseudo-2D Doyle-Fuller-Newman model is proposed to account for the different redox reactions that occur in dual-ion batteries and simulate the variation of average salt concentration in the electrolyte during charging and discharging. The model has been validated against galvanostatic cycling and electrochemical impedance spectroscopy experimental data from dual-ion batteries based on poly(2,2,6,6-tetramethyl-1-piperidinyloxy methacrylate) (PTMA). Such a model can be helpful to design practical dual-ion batteries that respect the constraints imposed by their working mechanism and maximize the obtainable capacity and energy density.