There is one limited time when you can get marginpars on both sides. If you are at the end of a paragraph in which you already placed a marginpar, you can \par\vspace{-\parskip}
, reverse the marginpar direction and do a marginpar on the other side.
With this particular implementation, I also allow some vertical shifting to the note, with my routine
\mnote[vertical shift]{l/r}{text}
Here is the example, where you see the note on both sides of the paragraph.
\documentclass{article}
\usepackage{xcolor}
\usepackage{stackengine}
\usepackage{lipsum}
\usepackage{MnSymbol}
\usepackage{graphicx}
\newcommand*\mnote[3][0pt]{%
\if l#2\reversemarginpar\def\pointer{\filledmedtriangleright}%
\def\stackalignment{r}\fi%
\if r#2\normalmarginpar\def\pointer{\filledmedtriangleleft}%
\def\stackalignment{l}\fi%
\marginpar{%
\topinset{%
\scalebox{1.5}{\textcolor{blue}{$\pointer$}}}{%
\belowbaseline[-1.5\baselineskip-#1]{%
\stackengine%
{-5pt}%
{\fcolorbox{blue}{white}{\parbox{1.8cm}%
{\vspace{3pt}\raggedright#3}}}%
{~\colorbox{white}{\sffamily Note}}%
{O}%
{l}%
{F}%
{F}%
{S}%
}%
}{%
3ex+#1}{%
-2ex}%
}%
}
\begin{document}
\lipsum[1-2]
this is a\mnote[2ex]{l}{some text for the mnote box} test%
\par\vspace{-\parskip}\mnote[1ex]{r}{both right and left}
\lipsum[3]
This is another test\mnote{r}{another mnote box test testing the
right side}
\lipsum[4]
\end{document}

memoir
class has a\sidepar
command that allows side notes, i.e. notes on the other side of the page than where the margin notes are. See this example. – jub0bs Mar 8 '13 at 23:11\sidebar
and the result seems to be exactly what I am looking for. However, I'm not keen on all the overhead that comes with memoir, nor on redoing my adjustments to the article class. The manual talks about a lot of work going into implementing\sidebar
- this isn't available as a package that can be loaded into article? – user27080 Mar 9 '13 at 9:40