With increasing knowledge specialization, disciplines continuously communicate and contest the boundaries of knowledge. Within the dynamic space of collaboration and communication between fields that has ensued, it is not clear whether these new boundaries are aligned to knowledge similarities, or to prestige overlaps. In this article, I explore the significant case example of college double major combinations to show: (1) how the configuration and availability of double major programs are linked to the epistemological and social aspects of the disciplines; and (2) the ways in which disciplinary communication is shaped by students' choices. Using a large administrative data set of undergraduate students, I show that students' choices of double majors maintain knowledge boundaries, as proposed by the epistemological essentialism approach. However, when students deviate from this dominant pattern, their choices align with the prestige of the field; this leads to a unique structure of combinations, but also creates hierarchies based on knowledge combinations.