  Many focal-reducer spectrographs, currently available at state-of-the art
telescopes facilities, would benefit from a simple refurbishing that could
increase both the resolution and spectral range in order to cope with the
progressively challenging scientific requirements but, in order to make this
update appealing, it should minimize the changes in the existing structure of
the instrument. In the past, many authors proposed solutions based on stacking
subsequently layers of dispersive elements and record multiple spectra in one
shot (multiplexing). Although this idea is promising, it brings several
drawbacks and complexities that prevent the straightforward integration of a
such device in a spectrograph. Fortunately nowadays, the situation has changed
dramatically thanks to the successful experience achieved through
photopolymeric holographic films, used to fabricate common Volume Phase
Holographic Gratings (VPHGs). Thanks to the various advantages made available
by these materials in this context, we propose an innovative solution to design
a stacked multiplexed VPHGs that is able to secure efficiently different
spectra in a single shot. This allows to increase resolution and spectral range
enabling astronomers to greatly economize their awarded time at the telescope.
In this paper, we demonstrate the applicability of our solution, both in terms
of expected performance and feasibility, supposing the upgrade of the Gran
Telescopio CANARIAS (GTC) Optical System for Imaging and
low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS).
