The objective of this work is to clarify the role of inertial effects
on the long-wavelength instabilities and dynamics of multilayer plane
Poiseuille flows. A weakly nonlinear set of equations governing the ev
olution of interfacial waves is derived and examined. Numerical simula
tions show that for the flows irregular interface profiles are generic
. For some systems a curious behaviour of interfaces is revealed: depe
nding on initial conditions, interfaces evolve into irregular or solit
on-like structures.