This can be done with subequations and two minipages, each with its own align environment. Note that to keep the vertical spacing equal in the two minipages I added a \vphantom fraction to each of the left equations so they had the same height as the right equations.

\documentclass{article}
\usepackage{amsmath,lipsum}
\begin{document}
\lipsum[1]
\begin{subequations}
\noindent\begin{minipage}{.4\textwidth}
\begin{align}
\nabla\cdot\vec E\vphantom{\frac{\partial \vec B}{\partial t}} &= \frac{\rho}{\epsilon_0}, \
\nabla\cdot\vec B\vphantom{\frac{\partial \vec B}{\partial t}} &= 0,
\end{align}
\end{minipage}% <-- % symbol necessary
\begin{minipage}{.6\textwidth}
\begin{align}
\nabla\times\vec E &= -\frac{\partial \vec B}{\partial t}, \
\nabla\times\vec B &= \mu_0\epsilon_0\frac{\partial \vec E}{\partial t}+\mu_0\vec j,
\end{align}
\end{minipage}
\end{subequations}
\vspace{\baselineskip}
\lipsum[2]
\end{document}
I set the left minipage at 40% of \textwidth, and the right at 60%, but you can adjust to your liking.