There are some packages in CTAN to have parallel texts, usually
in columns. You can see the complete list in www.ctan.org/topic/parallel.
The first example using the parallel package is to have just parallel columns:
\documentclass{article}
\usepackage[margin=2cm]{geometry}
\usepackage{parallel}
\usepackage{lipsum} % dummy text
\title{Parallel columns}
\author{You}
\begin{document}
\maketitle
\begin{Parallel}{.45\textwidth}{.45\textwidth}
\ParallelLText{\section{Heading\label{h}} \lipsum[1]}
\ParallelRText{\section*{Notes to heading \ref{h}} \lipsum[2]}
\end{Parallel}
\ParallelPar
\begin{Parallel}{.45\textwidth}{.45\textwidth}
\ParallelLText{\section{Heading 2\label{h2}} \lipsum[3]}
\ParallelRText{\section*{Notes to heading \ref{h2}} \lipsum[3]}
\end{Parallel}
\end{document}
You can convert this in facing pages simply adding the option [p] in the Parallel environment (of course, modifying also the width of the columns), but the end of the environment or \ParallelPar produce a change of page (each \Parallel...Text fill a complete page). In the next example is showed the problem:
\documentclass{article}
\usepackage[margin=4cm]{geometry}
\usepackage{parallel}
\usepackage{lipsum} % dummy text
\title{Facing pages}
\author{You}
\begin{document}
\maketitle
\begin{Parallel}[p]{\textwidth}{\textwidth}
\ParallelLText{\section{Heading One \label{h1}} \lipsum[1]}
\ParallelRText{\section*{Notes to heading \ref{h1}} \lipsum[2]}
\ParallelPar
\ParallelLText{\section{Heading One \label{h2}} \lipsum[1]}
\ParallelRText{\section*{Notes to heading \ref{h2}} \lipsum[2]}
\end{Parallel}
\begin{Parallel}[p]{\textwidth}{\textwidth}
\ParallelLText{\section{Heading One \label{h3}} \lipsum[1]}
\ParallelRText{\section*{Notes to heading \ref{h3}} \lipsum[2]}
\end{Parallel}
\end{document}
Edit
A completely different approach could be set manually the vertical position of each block of text in the even page with texpos package. Trial and error can be quite simplified with a macro hadling the synchrony of each pair of blocks. It must be checked that there are some separation between blocks and also that last block of each page does not extent to the lower margin, but this can be checked easily with the showboxes option and the showframes package. A example:
\documentclass[a4paper,twoside]{article}
\usepackage{afterpage}
% \usepackage{showframe} only for check block positions
\usepackage[margin=3cm]{geometry}
\usepackage{lipsum}
\usepackage{xcolor}
\usepackage{textpos} % use "showboxes" for check results
\setlength{\TPHorizModule}{.1\textwidth} % = 10% width
\setlength{\TPVertModule}{.01\textheight}% = 01% height
% Facing Double Block Macro
% Usage:
% \FDB{position}{section}{even text}{odd text}
% position = a number between 0 to 100 (or some less)
\newcommand\FDB[4]{
\begin{textblock}{10}(0,#1)
%\textblockcolour{cyan!10} % ugly, only for testing
\section{#2\label{#2}} #3 \bigskip
\end{textblock}
\afterpage{
\begin{textblock}{10}(0,#1)
% \textblockcolour{green!10} % ugly, only for testing
\section*{Annonations about section \ref{#2}}
#4\bigskip
\end{textblock}}}
\title{Facing pages with
\texttt{texdoc} and \texttt{afterpage}}
\author{You}
\begin{document}
\maketitle
\newpage
\FDB{0}{A title}{\lipsum[1]}{\lipsum[2]}
\FDB{25}{Another title}{\lipsum[3]}{\lipsum[4-5]}
\FDB{56}{One more title}{\lipsum[5]}{\lipsum[6]}
\FDB{78}{The last title of this page}{\lipsum[7]}{\lipsum[8]}
\newpage
% Annotations will be here
\newpage
\FDB{1}{The title of a newpage}{\lipsum[9]}{\lipsum[10]}
\end{document}
todonotespackage to put margin notes do the workaround. By default make orange boxes and lines but this can be changed easily. – Fran Jan 12 '13 at 21:33\ParallelParo renewing the environment split the pages. Moreover, this happen even deleting the six\newpagecommands in a redefinition of the internal\ParallelParTwoPages! (is showed in the manual) – Fran Jan 13 '13 at 09:58