Understanding how arousal shapes performance across competing tasks can provide insight into cognitive resource allocation under stress or high workload. This study investigated whether physiological arousal, indexed by pupil diameter, influences task prioritization during dual-task performance. Thirty adults completed concurrent auditory detection and mental arithmetic tasks under easy and difficult conditions. A trial-level trade-off index—reflecting the balance between arithmetic accuracy and auditory performance—was computed and examined across five individualized arousal levels. At the group level, arousal did not significantly affect task trade-offs, though participant-level quadratic trends suggested subtle, individualized inverted-U patterns between arousal and performance. Manipulation checks confirmed comparable mean pupil responses across task difficulty sessions. Overall, the results indicate that arousal–performance relationships are idiosyncratic rather than uniform, underscoring the importance of individual variability in cognitive performance under varying arousal states. Ongoing analyses using mixed-effects modeling aim to further characterize these individualized patterns and their implications for theories of arousal and performance.