Positive Plus One is the first large-scale mixed methods study of mixed HIV serostatus couples in Canada. We aimed to understand how biomedicalization i.e., a social process of commodification and expansion of the jurisdiction of medicine over health, influenced the everyday relationships of these couples. We completed 51 semi-structured interviews among a purposive sample of HIV-positive (n = 27) and HIV-negative (n = 24) partners in current or past mixed-serostatus relationships. Participants were recruited after completing an online survey where they consented to be re-contacted for qualitative interviews. Participants represented a diversity of sexual orientations, gender identities, and other sociodemographic characteristics. Drawing on inductive thematic analysis to identify patterns within couples, across serostatus and sexual orientations, we argue that everyday lives of mixed-serostatus couples were shaped by biomedical knowledge and enacted through routine adherence to obtain and maintain viral load undetectability. Our findings illustrated the importance of learning biomedical knowledge for mixed-serostatus couples in this study, the empowering influence of undetectable = untransmittable (UU) discourse, and its role in rendering HIV mundane through routine ART adherence. We introduce the concept of 'dual pharmaceutical citizenship' to underscore a process by which particular biopolitical and biomedical expectations are fulfilled in mixed-serostatus relationships. These findings have implications for people who do not readily accept or have access to biomedical knowledge, particularly when treatment-as-prevention frames a "right" and "wrong" approach to HIV management. Future studies should focus on couples where at least one partner does not readily accept or have access to biomedical knowledge.