  We compute the small-tau expansion up to the third order for the form factor
of two glued quantum star graphs with Neumann boundary conditions, by taking
into account only the most backscattering orbits. We thus show that the glueing
has no effect if the number of glueing edges is negligible compared to the
number of edges of the graph, whereas it has an effect on the $\tau^2$ term
when the numbers of glueing and non glueing edges are of the same order.
