A stratified two-phase gas-liquid flow confined in a channel is analyzed. Contrary to previous papers of the literature, where a parallel flow configuration is classically analyzed with a plug velocity profile in both fluids, in the present paper the flow is spatially developing, starting from a plug-plug profile at the channel entrance. The sudden change of interface boundary condition produces the flow development and the emergence of a single Kelvin-Helmholtz wave, whose formation and evolution are numerically studied. Numerical simulations are carried out by means of the volume of fluid (VOF) technique. The most physically relevant result is the nonlinear amplification of finite-amplitude entrance disturbances linked to the sudden change of inlet boundary condition, manifested as the formation and propagation of a single Kelvin-Helmholtz wave. In addition, a linear amplification of very small numerical artifacts, which arise around the interface region not yet reached by the traveling wave, is also discussed.