How to format Maxwell equations?

I think you are after aligned:

\documentclass{article}

\usepackage{amsmath}

\DeclareMathOperator{\Div}{div}
\DeclareMathOperator{\Rot}{rot}
\newcommand{\parder}[2]{\frac{\partial {#1}}{\partial {#2}}}

\begin{document}

\begin{align}
  \label{18.1:1}
  \begin{aligned}
    \Rot\vec{E} &=-\frac{1}{c}\parder{\vec{B}}{t},&
    \Div\vec{B} &=0,
    \\
    \Rot\vec{B} &=\frac{1}{c}\parder{\vec{E}}{t}
    +\frac{4\pi}{c}\,\vec{j},&
    \Div\vec{E} &=4\pi\rho_{\varepsilon}.
  \end{aligned}
\end{align}

\end{document}

Just use split:

\documentclass{article}
\usepackage{amsmath}

\DeclareMathOperator{\Div}{div}
\DeclareMathOperator{\Rot}{rot}

\newcommand\parder[2]{\frac{\partial #1}{\partial #2}}

\begin{document}
\begin{align}
\label{18.1:1}
\begin{split}
\Rot\vec{E} &=-\frac{1}{c}\parder{\vec{B}}{t},
\\
\Div\vec{B} &=0,
\end{split}
\\[2ex]
\label{18.1:2}
\begin{split}
\Rot\vec{B} &=\frac{1}{c}\parder{\vec{E}}{t}+\frac{4\pi}{c}\,\vec{j},
\\
\Div\vec{E} &=4\pi\rho_{\varepsilon }
\end{split}
\end{align}
\end{document}