  Some extensions beyond the Standard Model propose the existence of nearly
degenerate heavy sterile neutrinos. If kinematically allowed these can be
resonantly produced and decay in a cascade to common final states. The common
decay channels lead to mixing of the heavy sterile neutrino states and
interference effects. We implement non-perturbative methods to study the
dynamics of the cascade decay to common final states, which features
similarities but also noteworthy differences with the case of neutral meson
mixing. We show that mixing and oscillations among the nearly degenerate
sterile neutrinos can be detected as \emph{quantum beats} in the distribution
of final states produced from their decay. These oscillations would be a
telltale signal of mixing between heavy sterile neutrinos. We study in detail
the case of two nearly degenerate sterile neutrinos produced in the decay of
pseudoscalar mesons and decaying into a purely leptonic "visible" channel:
$\nu_h \rightarrow e^+ e^- \nu_a$. Possible cosmological implications for the
effective number of neutrinos $N_{eff}$ are discussed.
