Deficits in social skills are often exhibited in individuals with comorbid Down syndrome (DS) and autism spectrum disorder (ASD), and there is a paucity of research to help guide intervention for this population. In the present study, a multiple probe study across behaviors, replicated across participants, assessed the effectiveness of peer-delivered simultaneous prompting in teaching socials skills to adults with DS-ASD using visual analysis techniques and Tau-U statistics to measure effect. Peer-mediators with DS and intellectual disability (ID) delivered simultaneous prompting sessions reliably (i.e., > 80% reliability) to teach social skills to adults with ID and a dual-diagnoses of DS-ASD with small (Tau[subscript Weighted] = 0.55, 90% CI [0.29, 0.82]) to medium effects (Tau[subscript Weighted] = 0.75, 90% CI [.44, 1]). Statistical and visual analysis findings suggest a promising social skills intervention for individuals with DS-ASD as well as reliable delivery of simultaneous prompting procedures by individuals with DS.