I'm not sure whether you are producing these transcripts in an automated way. Also, whether your example transcript is representative of things in general (for example, you give 2 columns, which I assume to always be the case). I have assumed this in the solution presented below, and made some other assumptions about typesetting (like the font). Here is the basic idea...
We typeset the structure using a tabular that has 2 columns. You supply the tabular content in a form similar to that in your example (specifying the line break with a &), which is then printed twice. The first time, only the left column is shown, while the second time only shows the right column. At each print of the contents, the missing column is gobbled using a technique described in Easiest way to delete a column? The environ package allows to easily collect all the contents in the tabular and makes available (for multiple uses) in \BODY, enabling it to be processed twice.
In the example below I have used \ttfamily (mono-spaced Computer Modern font) to typeset the transcript, since it allows for easy horizontal alignment across lines. Also, I've used \phantom to adjust for padding some lines with the correct number of "blank letters". The array package provides the \newcolumntype macro for specifying the "gobble column" H. The new "line-breakable" tabular is contained with the created transcript environment:
\documentclass{article}
\usepackage{environ}% http://ctan.org/pkg/environ
\usepackage{array}% http://ctan.org/pkg/array
\newcolumntype{H}{>{\setbox0=\hbox\bgroup}c<{\egroup}@{}}% Gobble column
\NewEnviron{transcript}{%
\begin{tabular}{r@{}H}
\BODY
\end{tabular} \par
\begin{tabular}{H@{}l}
\BODY
\end{tabular}
}
\begin{document}
\ttfamily % Monospaced Computer Modern
tabular \textrm{environment:}
\begin{tabular}{r@{}l}
xxxxxxxxxxxxxxxxxxxxxxxxxxxxx&xxxxxxxxxxxxxx \
yyyyyyyyyyyyyyyyyyyyyyyyy&yyyyyyyyyyyyyyyyyy \
zzzzzzz&zzzzzz \
&\phantom{aa}aaa
\end{tabular}
transcript \textrm{environment:}
\begin{transcript}
xxxxxxxxxxxxxxxxxxxxxxxxxxxxx&xxxxxxxxxxxxxx \
yyyyyyyyyyyyyyyyyyyyyyyyy&yyyyyyyyyyyyyyyyyy \
zzzzzzz&zzzzzz \
&\phantom{aa}aaa
\end{transcript}
\end{document}

The above illustration showcases the regular tabular environment against transcript.
tabularenvironment could be useful here, but I don't quite understand your intent. Can you give us a more clear idea of the final layout desired? (perhaps uploading (or providing a link) to an image). – Gonzalo Medina Jun 23 '11 at 21:40