\begin{saveblock}{codebox}
	\begin{highlightblock}[linewidth=0.4\textwidth]
		% File: bibfile.bib
		@article {...
			...
		}
	
		@book {...
			...
		}
		...
	\end{highlightblock}
\end{saveblock}

\begin{saveblock}{codeboxB}
	\begin{highlightblock}[linewidth=0.6\textwidth]
		% File: document.tex
        \documentclass[a4paper]{article}
        \usepackage{natbib}
        \bibliographystyle{unsrtnat}

        \begin{document}
            This has been shown.
            \citep{babington}

            \bibliography{bibfile.bib}
        \end{document}
	\end{highlightblock}
\end{saveblock}

\begin{frame}{Minimum working example (natbib met backed bibtex)}
	% Voor natbib met backend BibTeX:
	
	\bigskip\medskip
	\begin{columns}
		\begin{column}{0.4\textwidth}
			\useblock{codebox}
		\end{column}%
		\begin{column}{0.6\textwidth}
			\useblock{codeboxB}
		\end{column}
	\end{columns}
\end{frame}