  In $R$-parity violating supersymmetry, the gravitino as the lightest
supersymmetric particle (LSP) is a good candidate for dark matter, with the
interesting characteristic to be detectable through $\gamma$-ray telescopes. We
extend this analysis considering an axino next-to-LSP (NLSP) as a coexisting
dark matter particle contributing with a detectable signal in the $\gamma$-ray
spectrum. The analysis is carried out in the framework of the $\mu\nu$SSM,
which solves the $\mu$ problem reproducing simultaneously neutrino data only
with the addition of right-handed neutrinos. We find that important regions of
the parameter space can be tested by future MeV-GeV $\gamma$-ray telescopes
through the line signal coming from the decay of the axino NLSP into
photon-neutrino. In a special region, a double-line signal from axino NLSP and
gravitino LSP is possible with both contributions detectable.
