The aim of the present study was to examine the relationship between response inhibition and working memory in 8-12-year-old children with attention-deficit/hyperactivity disorder (ADHD; n = 19), reading disorder (RD; n = 17), ADHD + RD (n = 21), and control children (n = 19). For the first time a within-task methodology was used to study the combined effect of both executive functions on a common measure of task performance in two often comorbid childhood disorders, ADHD and RD. We found evidence of an interaction between both domains, suggesting that they rely on a common pool of resources. In addition, we found that children with ADHD or RD were not more seriously affected by the combined load of both executive functions than children without ADHD or RD.