Beamer - gradual exposure in the same frame

You can use the \visible<overlay>{<stuff>} specification:

\documentclass{beamer}% http://ctan.org/pkg/beamer
\begin{document}
\begin{frame}
  \begin{itemize}
    \item \visible<1->{item 1}
    \item \visible<3->{item 2}
  \end{itemize}

  \vspace{1.5\baselineskip}

  \visible<2->{\begin{minipage}{0.4\textwidth}
    \begin{tabular}{c|c}
      table1 & trial \\
      \hline \hline
      1 & 2 \\
      3 & 4
    \end{tabular}
  \end{minipage}} \qquad
  \visible<4->{\begin{minipage}{0.4\textwidth}
    \begin{tabular}{c|c}
      table2 & trial \\
      \hline \hline
      5 & 6 \\
      7 & 8
    \end{tabular}
  \end{minipage}}
\end{frame}
\end{document}

The overlay <2-> means that the item will be visible from slide 2 onward, for example.

Instead of using minipage I recommend the using of columns/column. It allows the beamer specific settings.

\documentclass{beamer}

\begin{document}
\begin{frame}
 \begin{itemize}
  \item<1-> item 1
  \item<2-> item 2
 \end{itemize}

 \vspace{1.5\baselineskip}

  \begin{columns}[t]
  \begin{column}{.4\linewidth}<1->
    \begin{tabular}{c|c}
       table1 & trial \\\hline
       1 & 2 \\
       3 & 4
   \end{tabular}
  \end{column}
  \quad
  \begin{column}{0.4\textwidth}<2->
    \begin{tabular}{c|c}
       table2 & trial \\\hline
       5 & 6 \\
       7 & 8
   \end{tabular}
  \end{column}
  \end{columns}
\end{frame}
\end{document}

You may do this as follows.

\documentclass{beamer}

\begin{document}
\begin{frame}
        \begin{itemize}
            \item<1-> item 1
            \item<3-> item 2
        \end{itemize}

        \vspace{1.5\baselineskip}

        \onslide<2->{\begin{minipage}{0.4\textwidth}
            \begin{tabular}{c|c}
                table1 & trial \\
                \hline
                \hline
                1 & 2 \\
                3 & 4
            \end{tabular}
        \end{minipage} \qquad}
        \onslide<4>{\begin{minipage}{0.4\textwidth}
            \begin{tabular}{c|c}
                table2 & trial \\
                \hline
                \hline
                5 & 6 \\
                7 & 8
            \end{tabular}
        \end{minipage}}
\end{frame}
\end{document}