This study investigates the effect of co-CEO structure on asymmetric cost behavior. A firm’s cost behavior reflects managers’ decision making about resources, which can be influenced by various factors. One of them relates to a manager’s decision to inefficiently reallocate their company’s resources when sales decline in pursuit of their incentives for empire-building and disincentives for downsizing. These inefficient resource allocations may result in asymmetric cost behavior, and ultimately be harmful to a firm’s sustainability. We consider the co-CEO structure as an alternative corporate governance mechanism that prevents managers from making inappropriate decisions. By doing so, we investigate whether the degree of cost stickiness differs between co-CEO and single-CEO structures, and whether the former complements external governance mechanisms, particularly foreign ownership, in mitigating cost stickiness. We analyze data from Korean listed companies for 2000–2013, and find that the cost stickiness is lower in the co-CEO structure than in the single-CEO structure. Thus, the co-CEO structure works as an alternative corporate governance mechanism to control the agency problem by inducing mutual monitoring among co-CEOs. Furthermore, the reduction in cost stickiness is greater for firms with higher foreign ownership, indicating that the co-CEO structure complements external governance mechanisms.