  If more than one curvaton dominate the Universe at different epochs from each
other, curvature perturbations can be temporarily enhanced to a value much
larger than the observed one 10^{-5}. The traces of the enhancement may be left
as higher order correlation functions, that is, as non-Gaussianity, the
stochastic gravitational waves that are sourced by scalar-scalar mode
couplings, as well as the primordial black holes that are formed by the
gravitational collapse of the enhanced curvature perturbations. We first
confirm that such a temporal enhancement indeed occurs by solving the
linearized perturbation equations both numerically and analytically. We then
derive an analytic expression of the full-order curvature perturbation which
does not rely on the frequently used sudden decay approximation and is exact on
super-horizon scales. By using this analytic formula, we provide expressions of
the non-linearity parameters f_nl, tau_nl and g_nl. If both two curvatons
contribute to the final curvature perturbations, the strong non-Gaussianity
appears in the trispectrum rather than in the bispectrum. We also find a unique
consistency relation between tau_nl and g_nl without f_nl. By using the
second-order perturbation theory, we numerically show that the spectrum of the
induced gravitational waves has a plateau corresponding to duration of the
enhancement and such gravitational waves can be probed by ultimate-DECIGO and
space-based atomic interferometers. We finally calculate the abundance of the
primordial black holes and put a constraint on the amplitude of the enhanced
curvature perturbations.
