This paper examines the double dissociation hypothesis in light of the clinical phenotypes of two neurodevelopmental disorders, Developmental Language Disorder (DLD) and Williams syndrome (WS), within theoretical frameworks that posit an inherent cognitive architecture and treat language as an autonomous subsystem. Specifically, it outlines the double dissociation hypothesis, according to which two clinical groups exhibit contrasting patterns of impairment across cognitive domains, and sets out the predictions this hypothesis makes for DLD and WS. Drawing on (i) empirical findings on the linguistic and cognitive abilities of individuals with DLD and WS and (ii) recent developments in the clinical characterization of DLD, the paper critically evaluates the robustness of the double dissociation hypothesis as a framework for interpreting the clinical phenotypes of these disorders.