The contact between two membranes can be described by a system of variational
inequalities, where the unknowns are the displacements of the membranes and the
action of a membrane on the other one. We first perform the analysis of this
system. We then propose a discretization, where the displacements are
approximated by standard finite elements and the action by a
local postprocessing. Such a discretization admits an equivalent mixed
reformulation. We prove the well-posedness of the discrete problem and establish
optimal a priori error estimates.