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Nesting is not possible. Use simply the itemize environment and
\fromSlide{} for each \item, e.g.

% Here are slides 1-4
% Following items appear on slides 5 and 6
\begin{itemize}
\fromSlide{5}{\item Item no. 1}
\fromSlide{6}{\item Item no. 2}
\end{itemize}