# double column vs. single column text

Hello users.
I use Lyx 1.6.9 Memoir
I'm working on a book that requires some pages to be single column and some
to be double.
So according to something I saw somewhere, I ERTed \begin{multicols}{2} and
\end{multicols} and got the following error message:

LaTex Error: \begin{document} ended by \end{multicols}.

According to something else I read somewhere, I was supposed to insert
\usepackage{multicol} in the document preamble, which I did. Now I get the
following errors:

Missing \begin{document}
Environment multicols undefined
\begin{document} ended by \end{multicols}
Can only be used in preamble.

Can't seem to find any other info on this topic. Does anyone here have a
solution?

Richard Opheim