<p>Perovskite solar cells are rapidly emerging as promising candidates for next-generation photovoltaics, but their large-scale deployment is still limited by lead toxicity and long-term instability. To address these challenges, we numerically design and optimize a fully lead-free double-absorber perovskite solar cell that combines Rb₂LiInBr₆ as the top absorber and MASnI₃ as the bottom absorber using SCAPS-1D. This bandgap-complementary pair is selected to broaden solar spectrum utilization while leveraging the superior stability of Rb₂LiInBr₆ to compensate for the relatively lower robustness of MASnI₃. The proposed FTO/SnS₂/Rb₂LiInBr₆/MASnI₃/CuO/Au device architecture is systematically optimized by varying absorber thicknesses, electron and hole transport layers, defect densities, operating temperature, and metal work function. The best-performing configuration achieves an open-circuit voltage of 1.1371&#xa0;V, a short-circuit current density of 34.7112&#xa0;mA/cm², a fill factor of 82.21%, and a power conversion efficiency of 32.45% under standard AM 1.5G illumination. An MASnI₃ thickness of around 1&#xa0;μm is found to be optimal for balancing light absorption and charge collection, whereas higher defect densities and elevated temperatures lead to pronounced efficiency losses. In addition, the simulations show that a back metal work function of at least 5.10&#xa0;eV is essential for efficient hole extraction and high device performance. Overall, this work provides clear design guidelines for high-efficiency, lead-free double-absorber perovskite solar cells and highlights the Rb₂LiInBr₆/MASnI₃ combination as a strong candidate for environmentally benign, next-generation photovoltaic technologies.</p>