The combined effect of focussing and diffraction for a double crystal monochromator which is being designed for the Superconducting Wiggler beamline (SCOW) on ADONE. A double separated crystal system has been chosen; the possibility of obtaining focalization in two steps, by bending dynamical the crystals in order to obtain both tangential and sagittal focussing is studied. Non-focusing and semi focusingsystems have also been considered. The calculations have been performed with the ray tracing program SHADOW. On the basis of these results a 1:3 geometry for the monochromator position and estimate performances for the various focussing geometries is selected. The effect of anticlastic curvature is also considered.