Age-related neurodegenerative disorders like Alzheimer's disease (AD) and Parkinson's disease (PD) take an overwhelming toll on individuals and society. Traditionally, aggregation of a single protein (tau in AD and α-synuclein, or αS, in PD) was thought to be responsible for causing these different pathologies. However, there is increasing evidence that the pathologies of these two diseases overlap, and the individual proteins primarily associated with each disease promote each other's aggregation. Both tau and αS are intrinsically disordered proteins, lacking stable secondary structures under physiological conditions. Here, we complement ensemble aggregation studies with fluorescence correlation spectroscopy to investigate the interactions between tau and αS. Specifically, we measure interactions between full-length tau and constructs corresponding to its proline rich region (PRR) and microtubule binding domain (K18) with monomeric and aggregated forms of αS. We find only weak interactions between any of the tau variants and monomeric αS. Binding to αS aggregates, however, is enhanced relative to monomer, particularly by the PRR. The presence of the C-terminal tail of αS further amplifies this interaction. Interestingly, neither full-length nor truncated αS seeding has a significant impact on tau aggregation under the conditions of our experiments. The findings of our study draw attention to a central role of the PRR in mediating protein-protein interactions and underscores the complexity of the homotypic and heterotypic interactions underlying pathological protein aggregation.