  In most models of the dark sector, dark matter is charged under some new
symmetry to make it stable. We explore the possibility that not just dark
matter, but also the force carrier connecting it to the visible sector is
charged under this symmetry. This dark mediator then acts as a Double-Dark
Portal. We realize this setup in the \emph{dark mediator Dark matter} model
(dmDM), featuring a fermionic DM candidate $\chi$ with Yukawa couplings to
light scalars $\phi_i$. The scalars couple to SM quarks via the operator $\bar
q q \phi_i^* \phi_j/\Lambda_{ij}$. This can lead to large direct detection
signals via the $2\rightarrow3$ process $\chi N \rightarrow \chi N \phi$ if one
of the scalars has mass $ \lesssim 10$ keV. For dark matter Yukawa couplings
$y_\chi \sim 10^{-3} - 10^{-2}$, dmDM features a thermal relic dark matter
candidate while also implementing the SIDM scenario for ameliorating
inconsistencies between dwarf galaxy simulations and observations. We undertake
the first systematic survey of constraints on light scalars coupled to the SM
via the above operator. The strongest constraints are derived from a detailed
examination of the light mediator's effects on stellar astrophysics. LHC
experiments and cosmological considerations also yield important bounds.
Observations of neutron star cooling exclude the minimal model with one dark
mediator, but a scenario with two dark mediators remains viable and can give
strong direct detection signals. We explore the direct detection consequences
of this scenario and find that a heavy $\mathcal{O}(100)$ GeV dmDM candidate
fakes different $\mathcal{O}(10)$ GeV WIMPs at different experiments. Large
regions of dmDM parameter space are accessible above the irreducible neutrino
background.
