  A mean-field treatment is presented of a square lattice two-orbital-model for
${\rm BiS_2}$ taking into account intra- and inter-orbital superconductivity. A
rich phase diagram involving both types of superconductivity is presented as a
function of the ratio between the couplings of electrons in the same and
different orbitals (${\rm \eta= V_{XX}/V_{XY}}$) and electron doping $x$. With
the help of a quantity we call orbital-mixing ratio, denoted as $R(\phi)$, the
phase diagram is analyzed using a simple and intuitive picture based on how
$R(\phi)$ varies as electron doping increases. The predictive power of
$R(\phi)$ suggests that it could be a useful tool in qualitatively (or even
semi-quantitatively) analyzing multiband superconductivity in BCS-like
superconductors.
