It’s commonly necessary to present text in two languages ‘together’ on a page, or on a two-page spread. For this to be satisfactory, one usually needs some sort of alignment between the two texts.
The parallel package satisfies the need, permitting typesetting in two columns (not necessarily of the same width) on one page, or on the two opposing pages of a two-page spread. Use can be as simple as
\usepackage{parallel}
...
\begin{Parallel}{}{
Parallel can get terribly confused with colour changes, in
PDFTeX; the indefatigable Heiko Oberdiek has a patch for this
issue — the pdfcolparallel package, which maintains
separate colour stacks for the columns.
The parcolumns package can (in principle) deal with any number of columns: the documentation shows its use with three columns. Usage is rather similar to that of parallel, though there is of course a “number of columns to specify”:
\usepackage{parcolumns}
...
\begin{parcolumns}[]{3}
\colchunk{}
\colchunk{}
\colchunk{}
\colplacechunks
...
\end{parcolumns}
The ‹options› can specify the widths of the columns, whether to
place rules between the columns, whether to set the columns sloppy,
etc. Again, there are issues with colours, which are addressed by the
pdfcolparcolumns package.
The ledpar package is distributed with (and integrated with) the ledmac package. It provides parallel setting carefully integrated with the needs of a scholarly text, permitting translation, or notes, or both, to be set in parallel with the ‘base’ text of the document.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=parallel