documentclass{beamer}
usepackage{tikz} % for foreach

setbeamertemplate{sections/subsections in toc}[square]

begin{document}

section{Overview}
begin{frame}[plain]
    frametitle{contentsname}
    %https://tex.stackexchange.com/questions/109748/
    begin{columns}[t]
        begin{column}{.5textwidth}
            tableofcontents[sections={1-6}]
        end{column}
        begin{column}{.5textwidth}
            tableofcontents[sections={7-12}]
        end{column}
    end{columns}
end{frame}

foreach x in {1,...,11}{
    section{Section x}
    begin{frame}
    frametitle{insertsection}
        Test
    end{frame}
}

end{document}