Two versions from same tex file

Here's a solution using a flag. You can set the flag in the preamble, change it on the fly in the document if you wish.

With a little more work you could set the flag on the compile command line without having to edit the file.

\documentclass{article}

\usepackage{etoolbox}
\newtoggle{vtwo}

% set status globally in the preamble
\togglefalse{vtwo}

\newcommand{\secondversion}[1]{%
\iftoggle{vtwo}{%
#1
}
% else
{}
}

\begin{document}

...first simple version \secondversion{with some more comments is now
  second version}....

% change status locally if you wish
\toggletrue{vtwo}

...first simple version \secondversion{with some more comments is now
  second version}....

\end{document}