Add a $\pm$ to the label in order to distinguish two different cases
Use \tag
and manually step the equation
counter.
Explanation: \begin{equation}
steps up the counter; however \tag
will cause the counter to be stepped down.
\documentclass[twocolumn]{article}
\usepackage{amsmath}
\begin{document}
We have a double equation
\begin{equation}\label{double}
f^{\pm}(x)=\dots
\tag{\theequation$^{\pm}$}
\stepcounter{equation}
\end{equation}
We also have a normal equation
\begin{equation}\label{single}
g(x)=\dots
\end{equation}
We can reference equation~\eqref{double} and equation~\eqref{single}.
How about~{\let\pm=+\eqref{double}} and {\let\pm=-\eqref{double}}?
\end{document}
Note: twocolumn
is just to make a smaller picture.
An abstracted version that also works with hyperref
.
\documentclass[twocolumn]{article}
\usepackage{amsmath}
\usepackage{hyperref}
\protected\def\hpm{\ensuremath{^\pm}}
\newcommand{\pmnumber}{\tag{\theequation\hpm}\stepcounter{equation}}
\newcommand{\eqrefp}[1]{{\let\pm=+\eqref{#1}}}
\newcommand{\eqrefm}[1]{{\let\pm=-\eqref{#1}}}
\begin{document}
We have a double equation
\begin{equation}\label{double}\pmnumber
f^{\pm}(x)=\dots
\end{equation}
We also have a normal equation
\begin{equation}\label{single}
g(x)=\dots
\end{equation}
We can reference equation~\eqref{double} and equation~\eqref{single}.
How about~\eqrefp{double} and \eqrefm{double}?
\end{document}
You can cheat by redefining \pm
while referencing:
\documentclass{article}
\usepackage{amsmath}
\newcommand*\tagpm{\stepcounter{equation}\tag{\theequation\(\pm\)}}
\newcommand*\eqrefp[1]{{\def\pm{+}\eqref{#1}}}
\newcommand*\eqrefm[1]{{\def\pm{-}\eqref{#1}}}
\begin{document}
\begin{equation}\label{eqi}\tagpm
f^\pm(x)=\ldots
\end{equation}
\eqref{eqi}
\eqrefp{eqi}
\eqrefm{eqi}
\end{document}