While it is tempting to merge the RGS1 and RGS2 data, or data from different pointings, to provide a single spectrum with a signal-to-noise improvement over either individual spectrum, this is strongly discouraged since it results in data degradation.
The pointings of the two instruments are not identical, resulting in different
dispersion angles and wavelength scales. Separate response files are always
required for each unit. While it is possible to merge spectra and response files,
great care must be taken to account for different exposure times, background
subtractions, error propagation, and so on. However, the resulting response will
always have inferior resolution to the originals. It is therefore simpler and
more accurate to keep data from the two RGS units separate and use both sets to
fit one model in tandem. Users who nonetheless wish to merge spectra from
RGS1 and RGS2 can do so using the task rgscombine. Information on
this task can be found here:
http://xmm.esac.esa.int/sas/10.0.0/doc/rgscombine/index.html