  We present a scenario for the peculiar coexistence of charge fluctuations
observed in quasi-2D 1/4-filled organic conductors $\theta$-(BEDT-TTF)$_2X$ in
the quantum critical regime where the charge ordering is suppressed down to
zero temperature. The scenario is explored in the extended Hubbard model
including electron-phonon couplings on an anisotropic triangular lattice. We
find that the coexisting fluctuations emerge from two different instabilities,
the ``Wigner crystallization on lattice'' driven by the off-site Coulomb
repulsion and the charge-density-wave formation due to the nesting of the Fermi
surface, not from phase competition or real-space inhomogeneity. This mechanism
explains the contrastive temperature dependence of two fluctuations in
experiments.
