Utilizing the synergistic effects of energy transfer and alloy formation by using structurally similar polymer donors with complementary absorption spectra is a promising strategy for boosting the power conversion efficiency (PCE) of ternary organic solar cells (OSCs). However, efficient ternary OSCs with two polymer donors are not often reported due to the dearth of compatible donors. In the present work, we have strategically selected two polymer donors, J51 and PTB7-Th, that share a planar benzodithiophene (BDT) group in the backbone, which makes them compatible in the ternary blend. The absorption spectrum of J51 overlaps well with the spectral region with maximum solar radiation (450 nm–600 nm), thereby effectively harvesting the visible solar spectrum. Surface and interfacial energy values obtained from the contact angle measurements established the formation of the J51:PTB7-Th organic alloy, which is suitable for improving the quality of the active layer blend. Moreover, efficient energy transfer from J51 to PTB7-Th was also confirmed through a series of steady-state and time-resolved photoluminescence measurements. Ternary OSCs were fabricated using J51:PTB7-Th:PC70BM as the active layer. The presence of dual mechanisms resulted in an astounding PCE enhancement of 18% for the optimum ternary with 20% J51 content, mainly due to the improvement in JSC and FF. Photocurrent density versus effective voltage analysis indicated an improvement in the exciton dissociation efficiency from 91.69% to 95.40% and charge collection efficiency from 73.77% to 77.98% for the optimal ternary device compared to the host binary. Furthermore, impedance spectroscopy analysis revealed a reduction in bulk resistance and an increase in the recombination resistance in the optimum ternary device, explaining the observed improvement in JSC and FF.