Purpose: Both EGFR and PI3K-Akt signaling pathways have been used as therapeutically actionable targets, but resistance is frequently reported. In this report, we show that enrichment of the cancer stem cell (CSC) subsets and dysregulation of Notch signaling underlie the challenges to therapy and describe the development of bispecific antibodies targeting both HER and Notch signaling. Experimental Design: We utilized cell-based models to study Notch signaling in drug-induced CSC expansion. Both cancer cell line models and patient-derived xenograft tumors were used to evaluate the antitumor effects of bispecific antibodies. Cell assays, flow cytometry, qPCR, and in vivo serial transplantation assays were employed to investigate the mechanisms of action and pharmacodynamic readouts. Results: We found that EGFR/Notch targeting bispecific antibodies exhibited a notable antistem cell effect in both in vitro and in vivo assays. Bispecific antibodies delayed the occurrence of acquired resistance to EGFR inhibitors in triple-negative breast cancer cell line-based models and showed efficacy in patient-derived xenografts. Moreover, the EGFR/Notch bispecific antibody PTG12 in combination with GDC-0941 exerted a stronger antitumor effect than the combined therapy of PI3K inhibitor with EGFR inhibitors or tarextumab in a broad spectrum of epithelial tumors. Mechanistically, bispecific antibody treatment inhibits the stem cell-like sub-population, reduces tumor-initiating cell frequency, and downregulates the mesenchymal gene expression. Conclusions: These findings suggest that the coblockade of EGFR and Notch signaling has the potential to increase the response to PI3K inhibition, and PTG12 may gain clinical efficacy when combined with PI3K blockage in cancer treatment.