Research shows that a substantial proportion of children with Down syndrome (DS) also meet the clinical criteria for Autism Spectrum Disorder (ASD). Children with this dual diagnosis display a linguistic profile that includes significant language delays and language impairments which often differ from the impairments observed in each developmental disability (DD) separately. Given the challenges observed with language acquisition for children with DS-ASD, concerns might be raised regarding the outcomes and suitability of a bilingual environment for children with this dual diagnosis specifically. The aim of this research was to explore the language profiles of four children with DS-ASD. A multiple case-study approach was employed. Four children with a confirmed DS-ASD diagnosis who had received exposure to two languages (English and Welsh) were assessed on a range of cognitive and linguistic measures. Performance was compared to three control groups; bilinguals with DS, English monolinguals with DS and mental age-matched typically developing bilinguals. Assessments comprised of expressive and receptive language, phonological awareness, working memory and non-verbal cognitive abilities. Considerable variability was found in the cognitive and linguistic profiles of the case-study participants. Children with DS-ASD displayed similar language profiles to that of the bilingual and monolingual children with DS in the areas tested, although performance was generally lower than that of the TD bilingual children. Although substantial variability was found, participants were developing bilingual abilities in a similar trajectory to children with DS in line with the degree of exposure to each language. This research highlights the need to assess bilingual children with complex dual diagnoses with an individualistic approach and carefully consider how to appropriately assess and treat bilingual children within speech and language therapy provisions.