\documentclass{article}

\usepackage{agda}

\pagestyle{empty}

\begin{document}

\noindent Text.
\AgdaHide{
\begin{code}
  mutual
\end{code}}
\begin{code}
    postulate A : Set
\end{code}
More text.
\begin{AgdaMultiCode}
\begin{code}
  mutual
\end{code}
\begin{code}
    postulate B : Set
\end{code}
\end{AgdaMultiCode}
Even more text.

\end{document}