This study aimed to apply electromyographic techniques and distributional analyses to test whether an increase in the strength of stimulus-response mapping could explain the mechanisms underlying the joint Simon effect. Within a single protocol, participants performed a Simon task and a Go/NoGo task in isolation, and a joint Go/NoGo task with a co-actor (joint Simon task). Results showed that joint-action impairs cognitive control and shortened reaction time by impacting both pre-motor time and motor time. Joint-action induced a larger facilitation on pre-motor time of ipsilateral than contralateral associations. This potentiation of the spatial correspondence effect plausibly explains the larger Simon-like effect usually observed in the joint Go/NoGo task compared to that observed in the isolated Go/NoGo task. The propensity of making incorrect activations and their concentration among fast responses also increased when working co-actively. Together, these findings indicate that joint-action increases the strength of automatic response capture induced by the stimulus location, promotes the delivery of the stronger association in the behavioral repertoire of the individual, and reduces cognitive control.