While the quality and nature of a PhD students' relationship with their supervisors is widely regarded as pivotal for successfully completing their studies, the increasing use of multiple supervisors may challenge this relationship. This is the first study to use interviews with students and supervisors to explore students' motivation in such supervisory arrangements. Our study is framed by Self-Determination Theory (SDT) as a means for understanding factors that enable (or inhibit) individuals' motivation to learn, and Social Penetration Theory (SPT) for its perspectives on the development of relationships. Findings show that students' self-disclosure fosters relatedness and enhances autonomy and competence, with motivation and well-being as a function of their relational needs being satisfied. As such, complementarities between SDT and SPT provide more nuanced insights into the influence of relatedness on student motivation and well-being.