Individuals’ networks are multiplex—bundles of roles, interactions, and exchanges—in which the boundaries between work relationships and non-work relationships are often blurred, or integrated. Surprisingly, though, there is a paucity of research that explicitly integrates the work-nonwork literature and the social networks literature. In this paper, we advance theory on cross-domain multiplexity—multifaceted relationships that occupy a blended work–nonwork role space by superimposing work and nonwork interactions, roles, and exchanges. Specifically, we draw from work-family border theory to propose that cross-domain multiplex relationships represent a qualitatively unique form of multiplexity that involve distinct norms, drivers, and tensions. We advance the conversation around social network analytics and work-nonwork boundary research by presenting a theoretical model of cross-domain multiplex relationships that unpacks their components, as well as their unique antecedents, outcomes, and dynamics. We also review three methodological approaches that scholars can employ to analyze our propositions. Given recent theoretical and methodological advancements in both arenas, we propose that employing social network analytic methodology can inform how scholars theorize and design research around the work-nonwork interface, expand the methodological toolkit applied to these research questions, and resolve inconsistencies in whether multiplex relationships are enriching or depleting.