This article applies the perspective of "boundary work," an approach originating in science studies, to relations between the disciplines of Second Language Writing (SLW) and rhetoric and composition (R&C), especially to controversies surrounding the concept of translingualism. Boundaries both separate and connect, a dual potential to exacerbate tensions or to create opportunities for cooperation. Translingualism has sometimes been regarded by R&C as a radical innovation and by SLW as a distracting novelty, but a closer exploration shows both common historical roots and shared contemporaneous parallels in disciplines such as applied linguistics and second language acquisition. For WAC/WID, the translingual challenge may lead to a deconstruction of the L1/L2 binary and to the further rhetorization of correctness, as we find ways to help faculty help students negotiate language choices within a context of acceptance of their full linguistic repertoire and empowerment of their writerly choices.