  In the energy range from few TeV to 25 TeV, upper bounds on the dark matter
decay rate into high energy monochromatic neutrinos have recently become
comparable to those on monochromatic gamma-ray lines. This implies clear
possibilities of a future double "smoking-gun" evidence for the dark matter
particle, from the observation of both a gamma and a neutrino line at the same
energy. In particular, we show that a scenario where both lines are induced
from the same dark matter particle decay leads to correlations that can already
be tested. We study this "double monochromatic" scenario by considering the
complete list of lowest dimensional effective operators that could induce such
a decay. Furthermore, we argue that, on top of lines from decays into two-body
final states, three-body final states can also be highly relevant. In addition
to producing a distinct hard photon spectrum, three-body final states also
produce a line-like feature in the neutrino spectrum that can be searched for
by neutrino telescopes.
