In the paper we study the discrete spectrum of a pair of quantum
two-dimensional waveguides having common boundary in which a window of finite
length is cut out. We study the phenomenon of new eigenvalues emerging from the
threshold of the essential spectrum when the length of window passes through
critical values. We construct the asymptotics expansions for the emerging
eigenvalues with respect to small parameter which is the difference between
current length of the window and the nearest critical value. We also study the
behaviour of the spectrum when the length of the window increases unboundedly
and construct asymptotics expansions with respect to great parameter which is a
length of the window.