  We report on two sub-band transport in double gate SiO$_2$-Si-SiO$_2$ quantum
well with 14 nm thick Si layer at 270 mK. At symmetric well potential the
experimental sub-band spacing changes monotonically from 2.3 to 0.3 meV when
the total density is adjusted by gate voltages between $\sim 0.7\times 10^{16}$
$-3.0\times 10^{16}$ m$^{-2}$. The conductivity is mapped in large gate bias
window and it shows strong non-monotonic features. At symmetric well potential
and high density these features are addressed to sub-band wave function
delocalization in the quantization direction and to different disorder of the
top and bottom interfaces of the Si well. Close to bi-layer/second sub-band
threshold the non-monotonic behavior is interpreted to arise from scattering
from localized band tail electrons.
