The human perceptual-motor system is tightly coupled to the physical and informational dynamics of a task environment and these dynamics operate to constrain the high-dimensional order of the human movement system into low-dimensional, task-specific synergies. The aim of the current study was to determine whether synergistic processes constrain and organize the behavior of co-acting individuals. Participants sat next to each other and each used one arm to complete a pointer-to-target task. Using the uncontrolled manifold, the structure of joint-angle variance was examined to determine whether there was synergistic organization at the interpersonal or intrapersonal levels. The results revealed the motor actions performed were synergistically organized at both the interpersonal and intrapersonal levels. More importantly, the interpersonal synergy was found to be significantly stronger than the intrapersonal synergies. Accordingly, the results provide clear evidence that the action dynamics of co-acting individuals can become temporarily organized to form single synergistic two-person systems.