在框架上从一列切换到两列

Question

我正在撰写演示文稿，想使用以下布局：

顶部有一个横跨整个页面宽度的大框，下面有两个并排的框。

我的问题是上框和下框的左右边缘无法对齐。我已经尝试了各种选项，例如总宽度和文本宽度，但无法使其工作。

\documentclass{beamer}

\usecolortheme{orchid}

\begin{document} 
  \begin{frame} 
    \begin{block}{Single Column} \vspace{4em} \end{block} 
    \begin{columns} 
      \begin{column}{.5\textwidth} 
        \begin{block}{Two Columns} \vspace{4em} \end{block} 
      \end{column} 
      \begin{column}{.5\textwidth} 
        \begin{block}{Two Columns} \vspace{4em} \end{block} 
      \end{column} 
    \end{columns} 
  \end{frame} 
\end{document}

Answer 1

\documentclass{beamer}

\usecolortheme{orchid}

\newsavebox{\squaredblocktext}
\setbeamertemplate{block begin}{
    \par\vskip\medskipamount%
    \makebox[\dimexpr\textwidth-1.5ex\relax][l]{%
        \begin{beamercolorbox}[colsep*=.75ex]{block title}
            \usebeamerfont*{block title}\insertblocktitle%
        \end{beamercolorbox}}%
        \begin{lrbox}{\squaredblocktext}%
            \begin{minipage}[t]{\textwidth}%
                \ifbeamercolorempty[bg]{block body}{\vskip-.25ex}{\vskip-.75ex}\vbox{}%
            }

            \setbeamertemplate{block end}{
            \end{minipage}%
        \end{lrbox}%
        {\parskip0pt\par}%
        \ifbeamercolorempty[bg]{block title}{}
        {\ifbeamercolorempty[bg]{block body}{}{\nointerlineskip\vskip-0.5pt}}%
        \usebeamerfont{block body}%
        \makebox[\dimexpr\textwidth-1.5ex\relax][l]{%
            \begin{beamercolorbox}[colsep*=.75ex,vmode]{block body}%
                \usebox{\squaredblocktext}
            \end{beamercolorbox}%
        }\vskip\smallskipamount%
    }

\begin{document} 
  \begin{frame} 
    \begin{block}{Single Column} \vspace{4em} \end{block} 
    \begin{columns}[onlytextwidth, T]
      \begin{column}{.45\textwidth} 
        \begin{block}{Two Columns} \vspace{4em} \end{block} 
      \end{column} 
      \begin{column}{.45\textwidth} 
        \begin{block}{Two Columns} \vspace{4em} \end{block} 
      \end{column} 
    \end{columns} 
  \end{frame} 
\end{document}