Scintillation cross-correlation coefficients at two radio wavelengths are investigated on the basis of a shallow-modulated phase screen for several different spectral descriptions of the scattering medium. For the relevant simplification, a sufficiently large outer-scale dimension, it is argued that the normalized correlation coefficient is primarily dependent on spectral shape and wavelength separation and is independent of anisotropy and outer-scale dimension. Ionospheric and interplanetary scintillation measurements are compared with this result.