This paper deals with a renewable energy conversion system based on cross-flow water turbines operating in a river stream. Two twin vertical towers, each of which is composed of four such turbines, need to meet an imposed restriction between their angular positions (synchronization) in order to achieve improved hydrodynamic performances. The synchronization is achieved by a control approach, without using any power coupling (mechanical or electrical) between the two towers. As each tower is equipped with a permanent-magnet synchronous generator which delivers electrical power to the grid through a back-to-back power electronics converter, their rotational speeds can be controlled such that to achieve angular position synchronization. Being similar to the phase-locked loop devices used in electronics, the proposed method has been tested on a power hardware-in-the-loop simulator dedicated to cross-flow water turbines and proved good potential for being easily and effectively used in practice due to its simplicity and reliability.