This study investigates the relationship between the simultaneous appointment of an auditor for a supplier and its major customer, and the resulting audit efficiency. When the same auditor audits both companies, information pertaining to relevant economic activities within the supply chain can be shared with the audit team, potentially enhancing audit efficiency. However, if a supplier appoints a common auditor with opportunistic intentions, such as to obtain information about its major customer, the auditor's informational advantage may be exploited to increase bargaining power over the supplier, leading to inefficiency. Using a sample of 4,188 firm-years of KOSPI-listed firms from 2011 to 2022, this study's empirical results indicate that supply chain common auditors demonstrate relatively higher efficiency. Specifically, they are associated with lower audit fees and fewer audit hours, while maintaining higher audit quality. These findings provide empirical evidence of the enhanced efficiency of supply chain common auditors and offer valuable insights for regulators and stakeholders.