How to advance HALF of a beamer slide

Question

I need to give beamer presentations with a lot of bullet points. The audience often ask for extra time so they can copy them, and periodically I need to go back to the previous slide to refer to a statement near the end.

Q: Is there a way to advanced just HALF a slide, like this:

slide N:

(top half of slide N)

(bottom half of slide N)

NEXT SLIDE:

(bottom half of slide N)

(top half of slide N+1)

NEXT SLIDE:

(top half of slide N+1)

(bottom half of slide N+1)

Of course I want to do this without creating a new slide (or frame).

Note: I'm aware of the issues of putting too much content on one slide, going too far etc. For the purpose of this question let's stick to beamer. THANKS :)

Answer 1

OK, per request. Note that this is a very simple example, and you will be even better off if you just use <1-2> and the like for the items. However, for more complex examples this may pay off. In any case, I will be happy to remove this if it is not useful.

\documentclass{beamer}
\usepackage{tikzducks,tikzlings}
\newcommand{\topA}{\begin{itemize}
\item this is a duck: \tikz[baseline=2em]{\duck}
\end{itemize}}
\newcommand{\botA}{\begin{itemize}
\item this is a mouse: \tikz[baseline=2em]{\mouse}
\end{itemize}}
\newcommand{\topB}{\begin{itemize}
\item this is a koala: \tikz[baseline=2em]{\koala}
\end{itemize}}
\newcommand{\botB}{\begin{itemize}
\item this is a marmot: \tikz[baseline=2em]{\marmot}
\end{itemize}}

\begin{document}
\begin{frame}[t]
\frametitle{Ti\emph{k}Zlings: duck and mouse}

\topA

\botA

\end{frame}

\begin{frame}[t]
\frametitle{Ti\emph{k}Zlings: mouse and koala}
\botA

\topB
\end{frame}

\begin{frame}[t]
\frametitle{Ti\emph{k}Zlings: koala and marmot}

\topB

\botB

\end{frame}
\end{document}

This, which is conceptually very similar tp leandriis' suggestion yields almost the same result (except for no top alignment, which I fail to understand, but this is definitely my fault).

\documentclass{beamer}
\usepackage{tikzducks,tikzlings}

\begin{document}
\begin{frame}[t]
\frametitle{Ti\emph{k}Zlings: \only<1>{duck}\only<2>{mouse}\only<3>{koala} and 
\only<1>{mouse}\only<2>{koala}\only<3>{marmot}}
\begin{itemize}
\item<1> this is a duck: \tikz[baseline=2em]{\duck}
\item<1-2> this is a mouse: \tikz[baseline=2em]{\mouse}
\item<2-3> this is a koala: \tikz[baseline=2em]{\koala}
\item<3> this is a marmot: \tikz[baseline=2em]{\marmot}
\end{itemize}
\end{frame}
\end{document}

So I guess we will have to wait until @samcarter writes a real answer.