Within the astronomical field, many focal-reducer spectrographs that are currently available at state-of-the-art telescopes facilities, would benefit from a simple refurbishing that aims to increase both the resolution and spectral range. This kind of upgrade would cope with the progressively challenging scientific requirements, but, in order to make it appealing, it should minimize the changes in the existing structure of the instruments. In the past, many authors already tried to propose solutions based on stacking subsequently many dispersive elements and recording multiple spectra in one shot (multiplexing). Although this idea is very promising, it brings several drawbacks and complexities that prevent the straightforward integration of such a device in a spectrograph. Fortunately, today, the situation has changed dramatically, thanks to the availability of new materials such as the photopolymeric holographic films, that have proven their reliability in the fabrication of volume-phase holographic gratings (VPHGs) for astronomy. Thanks to the various advantages made available by these materials in this context, we propose an innovative solution for designing stacked multiplexed VPHG that is able to secure efficiently different spectra in a single shot. This will allow to increase resolution and spectral range enabling astronomers to greatly economize their awarded time at the telescope. In this proceeding, 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).