If a source was observed simultaneously with two or even more instruments it might be worth while to fit all taken spectra simultaneously, too. This can be done by merging the different count rate spectra into one single spectral table as well as the corresponding correction vectors into a single correction vector table. In both cases the general header has to be copied to the new table using COPY/HEADER and at least the descriptor NUM_INSTR has to be set to the actual number of used instruments. The names of the corresponding detector response matrices have to be entered into a catalogue. All this is done by the command MERGE/SPECTRA. Both commands MODEL/SPECTRUM and FIT/SPECTRUM can be used to fit simultaneously merged spectra. Of course, the corresponding names of the merge tables and the catalogue name containing the names of the detector response matrices have to be used. By using the command FIT/SPECTRUM the internal binning (command BIN/DETECTOR_RESPONSE) is always skipped. Before plotting the fit result the output table has to be sorted according to column :ENERGY.