Behavioral phenotypes of individuals with Williams syndrome and individuals with Down syndrome have been contrasted in relation to short-term memory. People with Down syndrome are stronger visuospatially and those with Williams syndrome are stronger verbally. We examined short-term memory, then explored whether dual-task processing further characterized behavioral phenotypes in 53 older adults with Down syndrome, 10 with Williams syndrome, and 39 controls. Short-term memory profiles generally conformed to those of younger individuals. Pegs placement and number repetition were performed singly and simultaneously. There were no etiology group performance differences on single tasks. During concurrent processing, all groups maintained single-task performance on pegs, but declined on number repetition. However, participants with Down syndrome declined more, suggesting relatively greater weakness in the dual-task processing component of executive function for this group.