\documentclass{article}
\renewcommand{\labelenumi}{(\Roman{enumi})}
\renewcommand{\labelenumii}{\Roman{enumi}--\arabic{enumii}}
\begin{document}
With the user modified label commands for the first two level of the
enumerate environment, the corresponding labels occure now as follows:
\begin{enumerate}
\item The labels for the first level are roman captal numerals enclosed by
parenthesis.
\begin{enumerate}
\item The labels for the second level are the roman numerals of the first
      level followed by the number of the second level, seperated by
      a medium dash.
\item Here is the next label of the second level.
\item Followed by the third label of the second level.
\end{enumerate}
\item And back now to the second label of the first level.
\begin{enumerate}
\item And again at the second level with the combined roman numeral and
      the actual label number of this level.
\item And so one for continously labels of this label
\item as demonstrated here for the third label
\end{enumerate}
\item An back now the the next label of the first level
\item before we leave the enumerate environment.
\end{enumerate}
\end{document}
