Topsharing, a specific form of collective leadership in which two co-leaders jointly share a single leadership position, has gained increasing popularity across diverse organizational contexts. Despite its growing popularity, empirical research on Topsharing remains limited, particularly regarding the individual and contextual factors that contribute to effective co-leadership. Building on the Knowledge, Skills, Abilities, and Other characteristics (KSAO) framework, the present study aimed to identify the antecedents of successful Topsharing and integrate them into a conceptual model that can systematically guide the evaluation, selection, and development of co-leaders. A mixed-methods approach across two studies was employed. Study 1 involved 17 semi-structured interviews with co-leaders from multiple sectors, analyzed using qualitative content analysis to identify key individual and contextual factors. Study 2 surveyed 116 participants to empirically validate the relevance of these factors. The combined findings demonstrate that effective co-leadership depends on an interplay of leadership-related knowledge, interpersonal skills, shared attitudes, and supportive organizational conditions. Key competencies include communication, adaptability, and self-reflection. Moreover, personality traits such as openness and empathy, as well as values like trust, team orientation, and a shared leadership mindset, were identified as critical. Both complementary and supplementary antecedents were relevant, with similarity in key individual factors between co-leaders particularly enhancing effectiveness. Contextual factors, including social support, organizational culture, and structured selection processes, were also found to play an important role. Overall, this research presents an empirically grounded, theory-driven model of individual and contextual antecedents for effective Topsharing. This work advances leadership research and offers practical guidance for organizations in selecting and developing co-leaders to enhance Topsharing effectiveness.