Here's a more automated solution that saves you the trouble of numbering by hand. The basic idea is to define a new kind of \item
called \combine
which combines n items into one, where n can be specified as an optional argument (default 2).
In order to deal with enumerations with different label formats (alpha, roman etc.) I've also added a command to set the label format for the combined labels. This can be done before any particular list, or once in the document if all the lists will require combined elements will be of the same sort.
One caveat: don't try to use \label
with the combined items.
\documentclass{article}
\usepackage{calc}
\makeatletter
\newcounter{tmp@cnt}
\newcommand*\@labelpunc{.}
% command to combine items: optional argument to specify the number to be combined
% \combine[<num>]<item>
%
\newcommand*\combine[1][2]{%
\refstepcounter{enumi}
\setcounter{tmp@cnt}{\value{enumi}}
\addtocounter{enumi}{#1-1}
\item[\thetmp@cnt--\theenumi\@labelpunc]}
% command to set the label format and punctuation
% \labeltype[<punc>]{<format>} where default punctuation is none; <format> is
% \alph \Alph \roman \Roman etc. Not needed for default enumerations
%
\newcommand*\labeltype[2][]{\gdef\@labelpunc{#1}\renewcommand\thetmp@cnt{#2{tmp@cnt}}}
\makeatother
\usepackage{enumitem} % for demonstration purposes (not required for the above commands)
\begin{document}
\begin{enumerate}
\item An item
\item Another item
\combine Two items combined
\item Another one
\combine[4]Four items combined
\end{enumerate}
\labeltype{\Roman}
\begin{enumerate}[label=\Roman*]
\item An item
\combine Two items combined
\item The last item
\end{enumerate}
\end{document}

enumitem
perhaps), since that way you can see which number you're on in the code. Also, if you decide to split #2 and #3 up later on, it won't mess up the subsequent numbering.a
andb
) at once. I normally just use comments above each\item
to track the number, butenumitem
looks interesting