  Topological flat bands at the Fermi level offer a promising platform to study
a variety of intriguing correlated phase of matter. Here we present band
engineering in the twisted orbital-active bilayers with spin-orbit coupling.
The symmetry constraints on the interlayer coupling that determines the
effective potential for low-energy physics of moir\'e electrons are
exhaustively derived for two-dimensional point groups. We find the line graph
or biparticle sublattice of moir\'e pattern emerge with a minimal $C_3$
symmetry, which exhibit isolated electronic flat bands with nontrivial
topology. The band flatness is insensitive to the twist angle since they come
from the interference effect. Armed with this guiding principle, we predict
that twisted bilayers of 2H-PbS$_2$ and CdS realize the salient physics to
engineer two-dimensional topological quantum phases. At small twist angles,
PbS$_2$ heterostructures give rise to an emergent moir\'e Kagom\'e lattice,
while CdS heterostructures lead to an emergent moir\'e honeycomb lattice, and
both of them host moir\'e quantum spin Hall insulators with almost flat
topological bands. We further study superconductivity of these two systems with
local attractive interactions. The superfluid weight and
Berezinskii-Kosterlitz-Thouless temperature are determined by multiband
processes and quantum geometry of the band in the flat-band limit when the
pairing potential exceeds the band width. Our results demonstrate twisted
bilayers with multi-orbitals as a promising tunable platform to realize
correlated topological phases.
