BACKGROUND: Communication within brain networks depends on functional connectivity. One promising approach to modulate such connectivity between cortical areas is dual-site transcranial alternating current stimulation (tACS), which non-invasively applies weak alternating currents to two brain areas. OBJECTIVES: In the current study, we aimed to modulate inter-regional functional connectivity with dual-site tACS to bilateral primary motor cortices (M1s) during bimanual coordination and, in turn, alter behaviour. METHODS: Using functional magnetic resonance imaging (fMRI), we recorded participants' brain responses during a bimanual coordination task in a concurrent tACS-fMRI design. While performing a slow and fast version of the task, participants received one of three types of beta (20 Hz) dual-site tACS over both M1s: zero-phase, jittered-phase or sham, in a within-participant, repeated measures design. RESULTS: While we did not observe any significant tACS effects on behaviour, the study revealed an attenuation effect of zero-phase tACS on interhemispheric connectivity. Additionally, the two active types of tACS (zero-phase and jittered-phase) differed in the task-related M1 connectivity with other motor cortical regions, such as premotor cortex and supplementary motor area. Furthermore, individual E-field strengths were related to functional connectivity in the zero-phase condition. CONCLUSIONS: Dual-site beta tACS over both M1s altered functional connectivity between motor areas. However, this effect did not translate significantly to the behavioural level in the presence of a restricted sample size. Future studies may thus integrate mechanistic measures, such as measures of interhemispheric inhibition, to strengthen causal interpretations.