Adjustable column size in paracol

You can define the \columnratio that you want.

\documentclass{article}
\usepackage{paracol}
\usepackage{lipsum}
\usepackage{lmodern}

\begin{document}
\begin{paracol}{2}
\lipsum[2]
\switchcolumn
\lipsum[2]
\end{paracol}

\columnratio{0.6}
\begin{paracol}{2}
\lipsum[2]
\switchcolumn
\lipsum[2]
\end{paracol}

\columnratio{0.4}
\begin{paracol}{2}
\lipsum[2]
\switchcolumn
\lipsum[2]
\end{paracol}
\end{document}