Partial automation enhances multitasking performance by reducing the overall attention demands of multitasking placed on human operators. However, there is a critical gap in researchers’ understanding of how to design effective partial automation in multitasking, particularly when it comes to determining the types of tasks that should be automated. This study explores the impact of partial automation on multitasking performance under different sensory modalities and task prioritizations. Our study utilized the Multi-Attribute Task Battery II (MATB-II) as its multitasking environment and employed a 2 \(\times \) 2 within-subjects design with two independent variables: sensory modality (cross-modality vs. intra-modality) and task priority (equal vs. unequal priority). The individual subtask and overall multitasking performances of 20 participants were measured. Results indicate that partial automation led to greater improvements in both types of performance under cross-modality than under intra-modality. These findings offer guidance for designing automation allocation strategies to enhance multitasking performance.