%% 
%%  Example 02-09-4 
%%  Copyright (C) 2016 Herbert Voss
%% 
%% ==== 
% Show page(s) 1
%% 
%% 
\documentclass[]{screxa}
\pagestyle{empty}
\setlength\textwidth{328.62788pt}
\setlength\parindent{0pt}
%StartShownPreambleCommands
\usepackage{amsmath}
\newcounter{mySubCounter}
\newcommand\twocoleqn[2]{%
  \setcounter{mySubCounter}{0}%
  \let\OldTheEquation\theequation
  \renewcommand{\theequation}{\OldTheEquation\alph{mySubCounter}}%
  \noindent%
  \begin{minipage}{.49\textwidth}
     \begin{equation}\refstepcounter{mySubCounter} #1 \end{equation}
  \end{minipage}\hfill
  \addtocounter{equation}{-1}%
  \begin{minipage}{.49\textwidth}
    \begin{equation}\refstepcounter{mySubCounter}  #2 \end{equation}
  \end{minipage}%
  \let\theequation\OldTheEquation}
%StopShownPreambleCommands
\begin{document}
\twocoleqn{y=f(x)}{y=f(z)}
\end{document}
