The paper examines parallel operation of two adjacent steam power plants (SPP) under coordinated reactive power (Q) - voltage control. A coordinated reactive power-voltage controller (CQVC) performs optimal coordination of the synchronous generators' Q outputs in order to maintain the voltage at a SPP HV busbars. When the plants are equipped with CQVC, they are both trying to control the same voltage, so the Q oscillations may occur due to high level of coupling. Before the commissioning of the second CQVC detailed laboratory testing was performed to determine under which conditions the oscillations may occur. The impact of all generators on HV busbars voltage is determined first. Then the worst case scenario is chosen for the most influential generators, i.e., when both CQVCs act simultaneously and independently. The laboratory test rig is developed for CQVC testing. The results showed that coordinated control of influential generators results in stable operation and ensures uniform Q share among involved generators which leads to better power system voltage support.