We examined the effects of individual versus joint action on a Simon task using motion tracking to explore the implicit cognitive dynamics underlying responses. In both individual and joint conditions, participants were slower to respond, and were differentially attracted to the distracter response location, when the spatial component of the stimulus was incompatible with the response location. When two people completed similar two choice tasks together, the results were not statistically different from the individual condition, even though the magnitude of the stimulus-response compatibility effect was slightly larger. Neither was there an increased effect when the partner had no stimulus-response conflict to resolve. We found no evidence for an action conflict when the responses of the two partners were different. These data imply that the literature regarding the Joint Simon task is still in the process of determining the relevant events that interact with and support joint action.