Antibody-drug conjugates (ADCs) have demonstrated transformative potential in oncology, but enhancing efficacy of ADC against low-antigen and heterogeneous tumors while overcoming resistance and maintaining safety remain a major challenge. Resistance to single-payload ADCs, particularly TOPO1i based ADCs, is often driven by payload-specific mechanisms such as enhanced DNA repair or upregulation of drug efflux pumps, limiting sustained clinical responses. Sequential administration of TOPO1i ADCs frequently fails to restore efficacy, underscoring the need for novel, multi mechanistic strategies. Topoisomerase I inhibitors (TOPO1i) induce DNA breaks that collapse replication forks, but their efficacy is constrained by rapid repair via PARP and ATR dependent pathways. DNA damage response inhibitors (DDRi), such as PARPi and ATRi, block these repairs, restoring TOPO1i sensitivity in resistant, SLFN11-deficient, or repair-proficient tumors. This combination enforces checkpoint failure and prevents fork repair, providing a strong rationale for synergistic antitumor activity while limiting the development and progression of acquired resistance. Although combining TOPO1 inhibition with DDRi has a strong mechanistic rationale, clinical application is constrained by overlapping and non-overlapping toxicities. To overcome these challenges, we developed site-specific dual-payload ADCs (dpADCs) to co-deliver TOPO1i and proprietary DDR inhibitors, including PARPi or ATRi that are not substrates for drug efflux pumps. This strategy leverages synthetic lethality by pairing TOPO1i-induced DNA damage with DDR blockade, selectively enhancing cytotoxicity in target-antigen expressing tumor cells. Key challenges in developing TOPO1i × DDRi dpADCs include optimizing DDRi properties suitable for ADC delivery, selecting payload ratios, maintaining conjugate homogeneity, ensuring linker stability, and mitigating systemic toxicity. By leveraging Sutro's cell-free expression platform, precise dual-conjugation technology, and hydrophilic tumor-selective beta-glucuronidase cleavable linkers, we engineered homogeneous, site-specific dpADCs with tunable drug-to-antibody ratios (DARs). These design features enhance solubility, stability, and pharmacokinetics while preserving payload synergy. We demonstrate that Sutro's site-specific TOPO1i × DDRi dual-payload ADCs (dpADCs) exhibit superior anti-tumor activity compared to single-payload ADCs, including in models resistant to TOPO1i ADCs. The targeted co-delivery of TOPO1i and DDRi has the potential to minimize off-target toxicity typically associated with systemic DDR inhibition, thereby broadening the therapeutic window and streamlining preclinical and clinical development compared to separate combination regimens.