In the rare cases when two or more sources appear in the RGS aperture, rgsproc should be instructed to calculate the required number of spectra and modify its treatment of the background. In all such cases encountered so far, such as the observation of the active stars YY Gem and Castor shown in Fig. 40, the spectra have been well enough separated so that their selection regions can be treated independently. The current tools do not allow simultaneous estimates of overlapping spectra. It is not possible initially to specify more than one source, so a step-by-step approach is required adding sources one at a time with rgssources and setting source and background parameters accordingly before rerunning rgsproc to recalculate the spectra. Having accurate SIMBAD  coordinates for the brighter YY Gem and weaker Castor (not forgetting Castor's high proper motion), the sequence of events might be as follows to make spectra for both stars and exclude both from the background
rgsproc withsrc=yes srclabel=YYGem \ srcstyle=radec srcra=113.655858 srcdec=+31.869386 rgssources srclist=P0123710201R1S004SRCLI_0000.FIT addusersource=yes \ label=Castor ra=113.649428 dec=+31.888276 rgssources srclist=P0123710201R2S005SRCLI_0000.FIT addusersource=yes \ label=Castor ra=113.649428 dec=+31.888276 rgsproc entrystage=4:spectra procsrcexpr='INDEX==3&&INDEX==4' \ exclsrcexpr='INDEX==3&&INDEX==4'