<p>The emergence of the gig economy has introduced a new cohort of workers who take on supplementary work (“side hustles”) alongside their full-time jobs. This concurrent engagement presents multifaceted challenges for multiple job holders (MJHs). To understand MJHs’ resource management between their full-time jobs and side hustles, we applied boundary theory to construct a dynamic mechanism model. Using a dataset from 82 MJHs across 10 working days, we used a multilevel model to investigate how different variations in resources post-side hustle influenced MJHs’ energy and engagement in their full-time jobs. We found that at the between-person level, variability in resources post-side hustle was negatively correlated with the respondents’ average level of energy in their full-time jobs. In contrast, at the within-person level, daily shifts in resources post-side hustle were positively associated with the respondents’ subsequent energy levels in their full-time jobs. Additionally, at both the within-person and between-person levels, energy in the full-time job served as a mediator between resources post-side hustle and full-time job engagement. Our study examined the dynamic impact of resource management during the transition between MJHs’ side hustles and full-time jobs. The insights gained highlight the potential of side hustles to enhance the effectiveness of employees in their full-time jobs.</p>