Using the multicol package and embedding your list in a multicols environment does what you want:
\documentclass{article}
\usepackage{multicol}
\begin{document}
\begin{multicols}{2}
\begin{enumerate}
\item a
\item b
\item c
\item d
\item e
\item f
\end{enumerate}
\end{multicols}
\end{document}