According to https://tex.stackexchange.com/a/24563/ using \setlength{\columnsep}{<width>} one can change the width of the space between the two column.
I have two related questions:
(1) How can I obtain the original width of \columnsep?
(2) How can I tell latex to multiply the original width by 2 or another number?
(1) To obtain the old value after changing it you must store it somewhere (before changing it).
(2) You can use 2\columnsep to multiply with 2.
Example code:
\documentclass[twocolumn]{article}
\usepackage{lipsum}
%%%%
\newlength\oldcolumnsep
\setlength{\oldcolumnsep}{\columnsep}
\setlength{\columnsep}{2\columnsep}
%%%%
\begin{document}
\noindent
\verb|\columnsep:| \the\columnsep\newline
\verb|\oldcolumnsep:| \the\oldcolumnsep\newline
\lipsum[1-5]
\end{document}
