multiespecget is a especget wrapper. This task has as input a set of EPIC event lists and a source and background expression in (RA,DEC) and it runs especget for each of the input files, produncing a set of spectra, responses and effective area files.
The withmultiEPICspec allows users to run epicspeccombine task to combine all the output files, spectra, responses and effective area, into a three individual files: source and background spectra and response files.
The procedure to combine the spectra files and the effective area and response matrices files it is explained in the SAS thread: ``Combining the spectra of the 3 EPIC cameras''
http://xmm.esac.esa.int/sas/current/documentation/threads/epic_merging.shtml