In this research, an integrated acid–base tandem co-sensitization approach was formulated to improve the photovoltaic efficiency of dye-sensitized solar cells (DSSCs) under both indoor and outdoor lighting conditions. Novel pyrimidine- fluorescent based organic co-sensitizers (AS-1–AS-4) were systematically combined with the Ru(II)-based N3 dye to investigate their synergistic effect on light harvesting, charge transfer, and long-term stability. The optimized tandem configuration (AS-1 (bottom) + N3 (top)) demonstrated a remarkable 68% improvement in overall efficiency compared to the single N3 reference cell, achieving 11.12% under AM 1.5G illumination and 22.02% under 1000 lx indoor light. This significant enhancement is attributed to complementary spectral absorption, efficient acid–base interfacial coupling, and suppressed charge recombination, as confirmed by UV–Vis, EIS, IPCE, and stability analyses. The findings demonstrate that the acid–base tandem co-sensitization technique offers a highly effective and promising route for developing next-generation dye-sensitized solar cells (DSSCs) with superior efficiency, stability, and performance under low-light conditions.