Background: Learning from video lectures with peers, that is, co-viewing video lectures, is a common mode of learning across a wide range of ages and topics in the information age. Objectives: The present study tested the effects of learners' motivation on co-viewing video lectures in terms of learning performance, mental effort, and interpersonal brain synchronization (IBS). Methods: The current study was quantitative experimental. In total, 142 participants were recruited and divided into 71 dyads to manipulate task motivation from three aspects: meaning, interest, and reward. Then, they were divided into three experimental conditions according to the level of task motivation: two low task motivation learners (LL condition), a combination of one low and the other high (HL condition), and two high task motivation learners (HH condition). Two participants in a dyad were asked to view a video lecture on a shared screen and not talk until the end of the video lecture, with both learners in full view. Simultaneously, we recorded the cortical hemodynamic activity of two participants in each dyad using functional near-infrared spectroscopy (fNIRS). Results and Conclusions: The group-level analysis showed that two high-task motivation learners had better learning performance, greater mental effort, and stronger IBS than a combination of one low and one high-task motivation and two low-task motivation. The results from the individual assessment indicated that pairing a person with low task motivation with another with high task motivation resulted in benefits for the low-task motivation learner in terms of their self-perceived learning. However, it negatively affected the performance of the high task motivation learner. Implications: First, high-task motivation learners were encouraged to co-view video lectures. Second, instructors were encouraged to intervene between their interactions by providing timely feedback to avoid losses for high-task motivation learners, as this composition benefits low-task motivation learners.