The fourth industrial revolution and the rise of collaborative robots (cobots) in industrial assembly lines are changing the traditional role of human operators in assembly tasks from following a sequence of operations to working together with a cobot and being able to intervene at different stages of the assembly process. Hence, it is important to focus on cognitive flexibility training, instead of on procedural skill training. The current research examined the effect of performing several task segments simultaneously on cognitive flexibility training in an assembly task. We used a novel 3D simulation system to train participants in three between-participants groups. The three groups together -- One Segment, Two Segments, and Nine Segments -- comprised 29 participants. Following training, participants were required to assemble the model including having to switch between different sequences. The results demonstrate that the Nine Segments group required significantly longer training time compared to the One Segment group. For the Two Segments group, the switching time was shorter and the mean number of errors after switching was higher compared to the other groups. To conclude, only performing the task in two segments simultaneously improved both the cognitive flexibility and the subjective feeling of competence regarding participants' cognitive flexibility.