I will start with examples in order to more easily explain what I'm after.
The first example shows how I would like four equations arranged. The vertical alignment of the equal signs in each row is perfect, of course, because each is a single line in an align structure. But, not all four equations can be numbered. The other down side is lack of control of the horizontal spread without using a parbox.
The second example shows how I would like them numbered, but sacrifices the vertical alignment of the equal signs in each row, because they each have different vertical heights.
The third example is almost perfect. By swapping two of the equations and aligning the parboxes by [b], in this arrangement, the heights match better, but up close you can see the second row is very slightly off. It's not bad and it's the current solution I'm using.
In the second and third examples, the horizontal spread is exactly how I want it, being controlled by the parbox widths.
Here's the code:
\documentclass[10pt,a5paper]{book}
\usepackage[paperheight=9.5cm,paperwidth=13cm, margin=6mm]{geometry}
\usepackage{amsmath}
\usepackage{mathtools}
\newcommand{\pd}{\partial}
\newcommand{\gep}{\epsilon}
\newcommand{\gm}{\mu}
\newcommand{\gr}{\rho}
\thispagestyle{empty}
\begin{document}
Example 1
\begin{subequations}
\begin{align}
\vec{\nabla}\times \vec{E} +\frac{\pd \vec{B}}{\pd t} & = 0 &
\vec{\nabla}\cdot\vec{B} & = 0 \label{eqn:hB} \\
\vec{\nabla}\cdot\vec{E} &= \frac{\gr}{\gep_0} &
\vec{\nabla}\times \vec{B} - \gm_0 \gep_0 \frac{\pd \vec{E}}{\pd t}
&= \gm_0 \vec{j}\label{eqn:ihB}
\end{align}
\end{subequations}
Example 2
\begin{subequations}
\parbox[c]{\textwidth*7/16}{
\begin{align}
\vec{\nabla}\times \vec{E} +\frac{\pd \vec{B}}{\pd t} & = 0 \\
\vec{\nabla}\cdot\vec{B} & = 0 \label{eqn:hB}
\end{align}}
\parbox[c]{\textwidth*7/16}{
\begin{align}
\vec{\nabla}\cdot\vec{E} &= \frac{\gr}{\gep_0} \\
\vec{\nabla}\times \vec{B} - \gm_0 \gep_0 \frac{\pd \vec{E}}{\pd t}
&= \gm_0 \vec{j}\label{eqn:ihB}
\end{align}}
\end{subequations}
Example 3
\begin{subequations}
\parbox[b]{\textwidth*7/16}{
\begin{align}
\vec{\nabla}\times \vec{E} +\frac{\pd \vec{B}}{\pd t} & = 0 \\
\vec{\nabla}\cdot\vec{E} &= \frac{\gr}{\gep_0}
\end{align}}
\parbox[b]{\textwidth*7/16}{
\begin{align}
\vec{\nabla}\cdot\vec{B} & = 0 \label{eqn:hB} \\
\vec{\nabla}\times \vec{B} - \gm_0 \gep_0 \frac{\pd \vec{E}}{\pd t}
&= \gm_0 \vec{j}\label{eqn:ihB}
\end{align}}
\end{subequations}
\end{document}
The AMS align stuff does horizontal alignment of multiple equations above each other in a column well, it also does vertical alignment of multiple equations beside each other in a row, but how does one get multiple equation numbers on one line?
Is there a solution that gives full control of all three aspects simultaneously: multiple equation numbers on one line, horizontal alignment and vertical alignment?
Addendum
As there is more than one answer already, I'm adding some notes here, so as not to have to repeat myself in comments.
I will use phantoms and other structural tweaks when nothing else will do and that happens more than occasionally, unfortunately. If the answer to my question is that tweaks are the only solution, then so be it. But it seems to me that creating an align structure that allows having more than one equation number horizontally would solve all of this with zero tweaks. Is anyone from AMS development listening?
I appreciate all the suggestions below, because in lieu of having a built in solution, tweaks is all I have left and it's good to see how others solve this stuff.
An extra note on the semantics of vertical and horizontal alignment: I have at least two software programs that use these alignment terms to refer to the direction in which you move the elements to obtain alignment. Hence, vertical alignment takes elements in a row and moves them up or down to line them up and horizontal alignment takes elements in a column and moves them left or right to line them up. It also makes sense to me that the basic direction of the elements themselves could be used to define these terms, that is, above each other in a vertical line and beside each other in a horizontal line, giving the opposite definition to the above. The former definition refers to the method, the latter refers to the result.
\vec{j}with\vec{\jmath}. In fine math typography, it's common to suppress the "dot" of the lettersiandjif the letters are also topped by "accent characters" such as hats, tildes, and arrows.