The present study aimed to step into two-person neuroscience by investigating the hemodynamic correlates of between-brain connectivity during joint task performance. To test this approach, wireless functional near-infrared spectroscopy (fNIRS) was used to record brain signals during performance of a dual n-back task simultaneously in paired players as compared to single players. Evaluating functional connectivity between the paired players' brains using wavelet transform coherence (WTC) analysis revealed (1) a significant increase in between-brain coherence during joint task performance as compared to baseline condition. These patterns were observed in two frequency bands, i.e. in the heart rate (HR) frequency and in low-frequency oscillations (LFOs). (2) Averaged hemodynamic responses revealed larger responses in total hemoglobin concentration changes [tHb] for the paired players as compared to the single players; in addition, within the paired players groups joint task performance revealed larger changes in [tHb] as compared to a rest period and to a baseline condition. (3) No increase in behavioral performance was found in the paired players as compared to the single players. Our findings designate fNIRS as suitable tool for monitoring interpersonal performances between two subjects. The results show that two-person performance leads to relevant and significant effects, which are detectable using between-brain connectivity analysis. Using this approach can provide additional insight into interpersonal activation patterns not detectable using typical one-person experiments. Our study demonstrates the potential of simultaneously assessing cerebral hemodynamic responses for various two-person experimental paradigms and research areas where interpersonal performances are involved.