  The basic shape of di-pion mass spectra in the two-pion transitions of both
charmonia and bottomonia states is explained by an unified mechanism based on
contributions of the $\pi\pi$, $K\overline{K}$ and $\eta\eta$ coupled channels
including their interference. The role of the individual $f_0$ resonances in
shaping the di-pion mass distributions in the charmonia and bottomonia decays
is considered.
