Deciphering how the brain enables humans to interact, coordinate, and learn from one another remains one of the most compelling challenges in contemporary cognitive neuroscience. Social interaction is a dynamic, reciprocal process. Over the past decade, hyperscanning research has consistently identified Inter-Brain Synchronization (IBS) as a neural signature accompanying successful cooperation, communication, joint attention, and social learning. However, the correlational nature of these findings leaves a critical question unresolved: Does IBS cause better social interaction, or does it merely reflect it? While traditional hyperscanning paradigms are powerful in revealing inter-brain neural dynamics "in the wild," they cannot on their own determine the direction of causality. This gap has motivated the emergence of Multibrain Stimulation (MBS)—a new generation of causal inference tools designed to actively manipulate neural coupling across individuals. MBS leverages non-invasive transcranial electrical stimulation (tES) to modulate neural activity simultaneously in two or more interacting brains. Unlike conventional tES applied to a single individual, MBS employs coordinated stimulation parameters, such as synchronized waveforms or matched frequencies, to directly perturb the neural mechanisms underlying social interaction. By providing an exogenous, precisely controlled intervention on IBS, MBS satisfies interventionist criteria for establishing causal relationships: researchers can test whether modifying inter-brain synchrony leads to predictable changes in behavior, communication, or shared understanding. This capability represents a fundamental methodological shift, transforming interpersonal neuroscience from a largely descriptive discipline into one capable of mechanistic inquiry. The biophysical underpinnings of MBS vary depending on the specific modality used. Transcranial alternating current stimulation (tACS) functions through cross-brain entrainment: when two individuals receive oscillatory currents matched in frequency and phase (e.g., theta-, beta-, or gamma-band stimulation), their endogenous neural rhythms tend to align with the exogenous signal and, consequently, with each other. This alignment effectively instantiates principles of the Communication Through Coherence (CTC) framework, which posits that coherent oscillations optimize information exchange by synchronizing periods of excitability across neural populations. Meanwhile, transcranial direct current stimulation (tDCS) exerts its influence by altering the excitability of targeted cortical regions in a polarity-dependent manner, thereby tuning the computational readiness of social-cognitive hubs such as the temporoparietal junction, superior temporal cortex, or inferior frontal gyrus. A growing body of empirical evidence demonstrates that such manipulations yield robust behavioral effects. In joint motor tasks, in-phase tACS enhances interpersonal coordination by aligning motor preparation dynamics, reducing temporal variability, and enabling individuals to anticipate each other"s actions more effectively. In communication and social learning contexts, MBS targeting high-order integrative regions promotes conceptual alignment, accelerates knowledge transfer, and supports more efficient encoding of shared representations. Notably, the effects of MBS often persist beyond the stimulation period, suggesting short-term plasticity in cross-brain networks. Post-stimulation improvements in synchronization and coordination indicate that MBS may temporarily recalibrate the neural architecture underlying social interaction. However, these benefits exhibit strong parameter specificity—precise phase relationships (e.g., 0° in-phase versus 180° anti-phase) and frequency matching are essential for generating reliable behavioral outcomes. Taken together, MBS represents a transformative step toward establishing the causal principles of human sociality and offers a new avenue for probing how multiple brains become functionally aligned during interaction.