The synchronverter control algorithm is a highly promising option for operating AC power inverters in future low-inertia power systems. Yet, as with conventional synchronous generators, the standard synchronverter algorithm only provides limited robustness guarantees. Therefore, we propose an additional control that confers the closed-loop system with global robustness in the Input-to-State Stability (ISS) sense. In this paper, such a control law is derived for the case of two identical synchronverters interconnected over dynamic power lines by using the Control Leonov Function (CLeF) framework. The control is illustrated by simulations.