  We consider a model with two gauge singlet fermionic WIMPs communicating with
the SM particles by a singlet scalar mediator via a Higgs portal. While the
light WIMP is stable and plays the role of the dark matter (DM) candidate, the
heavy partner is a short-lived WIMP without contribution to the current DM
relic density. Along with the coannihilation effects the heavy WIMP, acting as
a mediator in $t$- and $u$-channel DM annihilation cross sections, has a
significant effect in finding the viable parameter space against the direct
detection constraints provided by XENON1t and LUX experiments. This is an
extension to the minimal singlet fermionic DM model whose entire parameter
space (except a resonance region) excluded by the latest direct detection
experiments. It is found out that there are viable regions in the parameter
space which evade direct detection upper bounds and respect the observed DM
relic density by WMAP/Planck. We also found that the Fermi-LAT upper limits on
the DM annihilation cross section into $b\bar b$ can exclude small regions of
the viable parameter space which elude direct detection experiments. This model
exemplifies a case within the WIMP paradigm whose DM candidate can escape
direct detection experiments nontrivially. Such models are interesting to be
studied in collider experiments like the LHC.
