In order to understand the so-called "social brain," we need to monitor social interactions in face-to-face paradigms. Near-infrared spectroscopy (NIRS) is a promising technique to achieve this goal. We investigate the neuronal underpinnings of sharing a task in a proper social context. We record cortical activity by means of NIRS, while participants perform a joint Simon task. Different from other hemodynamic techniques, NIRS allows us to have both participants sit comfortably close to each other in a realistic and ecological environment. We found higher activation in the sensorimotor cortex while processing compatible trials as compared to incompatible ones referring to one's own action alternative. Strikingly, when the participant was not responding because it was the turn of the other member of the pair, the inferior parietal was activated. This study provides twofold findings: first, they suggest that the joint Simon effect relies more on shared attentional mechanisms than a proper mapping of the other's motor response. Second, they highlight the invaluable contribution NIRS can afford to social neuroscience in order to preserve ecological and naturalistic settings.