  We report on theoretical and experimental investigations of a novel
hysteresis effect that has been observed on the magnetoresistance of
quantum-Hall bilayer systems. Extending to these system a recent approach,
based on the Thomas-Fermi-Poisson nonlinear screening theory and a local
conductivity model, we are able to explain the hysteresis as being due to
screening effects such as the formation of ``incompressible strips'', which
hinder the electron density in a layer within the quantum Hall regime to reach
its equilibrium distribution.
