The development of accessory breast tissue, which is found anywhere along the milk line, is attributed to the failure of milk line remnants to regress during embryogenesis. Primary tumors may arise from any ectopic breast tissue. Accessory breast cancer occurring concurrently with primary invasive breast cancer is extremely rare. Two such cases were reported in this article. One was a 43-year-old Chinese female who exhibited bilateral breast cancer (invasive ductal carcinoma, not otherwise specified, IDC-NOS) and an accessory breast carcinoma (IDC-NOS) incidentally identified in her left axilla. The ectopic breast tissue in her right axilla presented with adenosis. The patient was surgically treated, followed by postoperative docetaxel epirubicin (TE) chemotherapy. The second case was a 53-year-old Chinese female with bilateral breast cancer (apocrine carcinoma) accompanied by an accessory breast carcinoma (IDC-NOS) in her right axilla that was also incidentally identified. The patient was surgically treated after three doses of cyclophosphamide epirubicin docetaxel (CET) neoadjuvant chemotherapy, followed by adjuvant chemotherapy of the same regimen.