Two subtitle lines after the main title in the beamer presentation slide

Question

I would like to use two subtitle lines after the main title in the presentation slides using beamer. For example, I am using the following code:

\documentclass[final]{beamer}
\usetheme{Rochester}

\begin{document}

\begin{frame} 
\frametitle{\centerline{Main Title}}
\framesubtitle{\centerline{Subtitle Line 1}}
\framesubtitle{\centerline{Subtitle Line 2}}
\end{frame}

\end{document} 

I expect to obtain two subtitle lines. However, I get only one subtitle given in the second \framsubtitle command. The output is shown below:

How can I add two subtitle lines in the slide so that the Subtitle Line 1 also appears before the Subtitle Line 2. Further, is there any way to change the font sizes of the title and subtitle slides without modifying the beamer or any other files.

Apologies for basic questions.

Thanks in advance for help.

A.

Answer 1

Here's a hacky way with \raisebox and \parbox. Change the line with Subtitle Line 2 to:

\framesubtitle{\centerline{
  \raisebox{-4pt}{
    \parbox{\textwidth}{
      \centering Subtitle Line 2\\
      \scriptsize Subtitle Line 3
}}}}

This is the output:

Adjust the -4pt in the \raisebox to move the two lines up or down. Here's a link to local options for changing font size, which I've used for the third line above.