In this study, the authors examine the language practices of first- and second-year college students as they build disciplinary literacies within a cocurricular engineering leadership program. Conducted by two teacher educators and literacy/biliteracy researchers situated in the highly diverse U.S.-Mexico borderlands, the study examines the role of disciplinary language and literacy development among 12 "transfronterizo" (border-crossing) engineering students who crossed the U.S.-Mexico border on a regular basis, with a particular focus on the experience of two students. Drawing on systemic functional linguistics as well as sociocultural theories of literacy/biliteracy, the authors illustrate the ways in which these students moved fluidly between Spanish and English, as well as multiple registers and modalities, to make sense of engineering concepts. Findings build on recent studies of "transfronterizo" literacies and shed light on pedagogical practices that encourage students' use of their full linguistic repertoires in order to develop disciplinary literacy practices in engineering.