This chapter dismantles “dual diagnosis” as a category error. Rather than two disorders stacked inside one person, I argue for co-emergence: layered, relational responses to pain that cannot be disaggregated without harm. I examine the additive illusion, the grammar of labels that becomes structure, and the limits of comorbidity thinking. A stratified account replaces binary sorting and reframes assessment questions: not “Which came first?” but “What conditions made these responses necessary, and what needs to change now?” The chapter points toward integrated design anchored in ontology, not convenience.