  We examine how a scenario of coexisting two-particle dark mater can be
realized in the extended seesaw model, which we have proposed previously to
accommodate small neutrino masses and low scale leptogenesis with an
introduction of singlet Majorana neutrino $S$ and singlet scalar $\phi$. We now
impose the discrete symmetry $Z_2 \times Z_2^{\prime}$ and introduce new
renormalizable interaction terms with a new heavy singlet scalar particle
$\Phi$ so as for previously introduced $S$ and $\phi$ to be doubly coexisting
dark matter candidates. Depending on the mass spectrum of the two dark matter
candidates, the annihilation process either $SS\longrightarrow \phi \phi$ or
$\phi\phi\longrightarrow SS$ is of particular interest because the annihilation
cross sections for the processes can be so large that the relic abundance of
decaying particle should get lowered, which in turn makes the constraints on
its parameter space relaxed, compared with the case of one and only one dark
matter candidate. We discuss the implications of the dark matter detection
through the scattering off the nucleus of the detecting material on our
scenarios for dark matter candidates. We also study the implications for the
search of invisible Higgs decay at LHC, which may serve as a probe of our
scenario for dark matter.
