  Within a multicomponent dark matter scenario, novel gamma-ray signals may
arise from the decay of the heavier dark matter component into the lighter. For
a scalar dark sector of this kind, the decay $\phi_2\rightarrow\phi_1 \gamma$
is forbidden by the conservation of angular momentum, but the decay $\phi_2
\rightarrow \phi_1 \gamma\gamma$ can have a sizable or even dominant branching
ratio. In this paper we present a detailed study of this decay channel. We
determine the width and photon energy spectrum generated in the decay,
employing an effective theory approach, and in UV complete models where the
scalar dark matter components interact with heavy or light fermions. We also
calculate limits on the inverse width from current data of the isotropic
diffuse photon flux, both for a hierarchical and a degenerate dark matter
spectrum. Finally, we briefly comment on the prospects of observing the
diphoton signal from sneutrino decay in the minimal supersymmetric standard
model extended with right-handed neutrino superfields ($\tilde{\nu}$MSSM).
