  Let $\Omega\subset \mathbb{R}^N$ ($N\geq 3$) be an open domain which is not
necessarily bounded. The sharp constant and extremal functions to the following
kind of double-variable inequalities $$ S_{\alpha,\beta,\lambda,\mu}(\Omega)
\Big(\int_\Omega \big(\lambda \frac{|u|^{2^*(s)}}{|x|^s}+\mu
\frac{|v|^{2^*(s)}}{|x|^s}+2^*(s)\kappa \frac{|u|^\alpha
|v|^\beta}{|x|^s}\big)dx\Big)^{\frac{2}{2^*(s)}}$$ $$\leq \int_\Omega
\big(|\nabla u|^2+|\nabla v|^2\big)dx$$ for $(u,v)\in
{\mathscr{D}}:=D_{0}^{1,2}(\Omega)\times D_{0}^{1,2}(\Omega)$ will be explored.
Further results about the sharp constant $S_{\alpha,\beta,\lambda,\mu}(\Omega)$
with its extremal functions when $\Omega$ is a general open domain will be
involved. For this goal, we consider the following elliptic systems involving
multiple Hardy-Sobolev critical exponents: $$\begin{cases} -\Delta u-\lambda
\frac{|u|^{2^*(s_1)-2}u}{|x|^{s_1}}=\kappa\alpha
\frac{1}{|x|^{s_2}}|u|^{\alpha-2}u|v|^\beta\quad &\hbox{in}\;\Omega, -\Delta
v-\mu \frac{|v|^{2^*(s_1)-2}v}{|x|^{s_1}}=\kappa\beta
\frac{1}{|x|^{s_2}}|u|^{\alpha}|v|^{\beta-2}v\quad &\hbox{in}\;\Omega, (u,v)\in
\mathscr{D}:=D_{0}^{1,2}(\Omega)\times D_{0}^{1,2}(\Omega), \end{cases}$$ where
$s_1,s_2\in (0,2), \alpha>1,\beta>1, \lambda>0,\mu>0,\kappa\neq 0,
\alpha+\beta\leq 2^*(s_2)$. Here, $2^*(s):=\frac{2(N-s)}{N-2}$ is the critical
Hardy-Sobolev exponent. We mainly study the critical case (i.e.,
$\alpha+\beta=2^*(s_2)$) when $\Omega$ is a cone (in particular,
$\Omega=\mathbb{R}_+^N$ or $\Omega=\mathbb{R}^N$). We will establish a sequence
of fundamental results including regularity, symmetry, existence and
multiplicity, uniqueness and nonexistence, {\it etc.}
