  We consider Laplacian in a straight planar strip with Dirichlet boundary
which has two Neumann ``windows'' of the same length the centers of which are
$2l$ apart, and study the asymptotic behaviour of the discrete spectrum as
$l\to\infty$. It is shown that there are pairs of eigenvalues around each
isolated eigenvalue of a single-window strip and their distances vanish
exponentially in the limit $l\to\infty$. We derive an asymptotic expansion also
in the case where a single window gives rise to a threshold resonance which the
presence of the other window turns into a single isolated eigenvalue.
