We show that the solution of the radiative transfer equation (RTE) through a line of sight that pierces several times two alternate atmospheres can be obtained in terms of the solutions of the RTE through both single atmospheres separately considered. This also applies to the response functions of the observed Stokes spectrum to perturbations of the physical quantities. The analytic solution of the RTE in case that the single atmospheres are Milne-Eddington is presented. The simplification of the solution in the case of a longitudinal or transversal (with constant azimuth) magnetic field is presented as well. Finally, as a numerical example, we synthesize the Stokes I- and V-spectrum emerging from a thin magnetic flux tube, achieving a considerable decrease in computation time with respect to conventional integrations and without loss of accuracy.