A central issue in the study of joint task performance has been one of whether coacting individuals perform their partner’s part of the task as if it were their own. The present study addressed this issue by using joint task switching. A pair of actors shared two tasks that were presented in a random order, whereby the relevant task and actor were cued on each trial. Responses produced action effects that were either shared or separate between co-actors. When co-actors produced separate action effects, switch costs were obtained within the same actor (i.e., when the same actor performed consecutive trials) but not between co-actors (when different actors performed consecutive trials), implying that actors did not perform their co-actor’s part. When the same action effects were shared between coactors, however, switch costs were also obtained between co-actors, implying that actors did perform their co-actor’s part. The results indicated that shared action effects induce task-set sharing between co-acting individuals.