AIMS: For six neutron-star atoll sources (4U 1608-52, 4U 1636-53, 4U 0614+09, 4U 1728-34, 4U 1820-30 and 4U 1735-44) we investigate the relationship between the observed fractional rms amplitudes of the twin kHz QPOs. We discuss whether this displays features that could have a physical meaning in terms of the proposed QPO models. RESULTS: For each of the six sources, we find that there is a point in the R domain around which the amplitudes of the two twin kHz QPOs are the same. We find such a point located inside a narrow interval R=1.5 +-3%. Further investigation is needed in the case of two sources to explore this finding, since we have not determined this point in Set II. There is evidence of a similar point close to R = 1.33 or R = 1.25 in the foursources.