The lateral prefrontal cortex and the basal ganglia are known to be important for response selection processes, also in dual-task situations. However, response selection in dual-task situations can be achieved using different modes ranging from a parallel selection to a more serial selection of responses. Nothing is known whether differences in these processing modes during dual-tasking have distinct functional neuroanatomical correlates. In this fMRI study we analyzed performance in a psychological refractory period paradigm. In this paradigm we design a PRP task where we vary the frequency of short and long stimulus onset asynchronies between the two tasks. Using mathematical constraints we interpret the effects of this manipulation with respect to processing modes ranging from more serial to more parallel response selection. Contrastingly these blocks showed that response selection in dual-tasking under the constraint of more parallel processing is mediated by mechanisms operating at the striatal level, while response selection under the constraint of more serial processing is mediated via mechanisms operating in the lateral prefrontal cortex. The results suggest that lateral prefrontal and striatal regions are ‘optimized’ for a certain processing modes in dual tasking.