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. METHOD: We consider the difference in rms amplitude between the upper and lower kHz QPOs, as a function of the frequency ratio R. We compare two data sets. Set I is a collection taken from published data. Set II has rms amplitude values obtained by automatic fitting of continuous segments of RXTE-PCA observations.