This paper investigates the interaction of a small number of modes in the two-fluid Kelvin-Helmholtz instability at the nonlinear regime by using a two-dimensional hydrodynamic code.This interaction is found to be relatively long range in wave-number space and also it acts in both directions,i.e.short wavelengths affect long wavelengths and vice versa.There is no simple equivalent transformation from a band of similar modes to one mode representing their effective amplitude.Three distinct stages of interaction have been identified.