Longtable and page break
I don't think that's possible with longtable, or any other tabular environment for that matter, because the material inside the tabular cells are TeX boxes, which by definition cannot be broken.
I suggest you use parcolumns
instead, which has been designed to typeset several columns next to each other for multilingual texts. Here's an example:
\documentclass{article}
\usepackage{lipsum} % Used to produce long text
\usepackage{parcolumns}
\parindent=0pt
\begin{document}
\begin{parcolumns}[colwidths={2=0.74\textwidth},rulebetween,sloppy]{2}
\colchunk[1]{\textbf{ID}}
\colchunk[2]{\texttt{UC\_01\_01}}
\colplacechunks
\colchunk[1]{\textbf{Name}}
\colchunk[2]{Password recovery}
\colplacechunks
\colchunk[1]{\textbf{\textit{Macro}-UC}}
\colchunk[2]{Registration (\texttt{UC\_01})}
\colplacechunks
\colchunk[1]{\textbf{Actors}}
\colchunk[2]{\texttt{ACTOR\_02}}
\colplacechunks
\colchunk[1]{\textbf{Preconditions}}
\colchunk[2]{\lipsum[2-4]}
\colplacechunks
\colchunk[1]{\textbf{Main flow}}
\colchunk[2]{Some itemize list here!}
\colplacechunks
\colchunk[1]{\textbf{Postconditions}}
\colchunk[2]{\lipsum[5-9]}
\colplacechunks
\end{parcolumns}
\end{document}
Just to record parallel
package:
\documentclass{article}
\usepackage{lipsum} % Used to produce long text
\usepackage{parallel}
\parindent=0pt
\begin{document}
\begin{Parallel}[v]{0.4\textwidth}{0.55\textwidth}
\ParallelLText{\textbf{ID}}
\ParallelRText{\texttt{UC\_01\_01}}
\ParallelPar
\ParallelLText{\textbf{Name}}
\ParallelRText{Password recovery}
\ParallelPar
\ParallelLText{\textbf{\textit{Macro}-UC}}
\ParallelRText{Registration (\texttt{UC\_01})}
\ParallelPar
\ParallelLText{\textbf{Actors}}
\ParallelRText{\texttt{ACTOR\_02}}
\ParallelPar
\ParallelLText{\textbf{Preconditions}}
\ParallelRText{\lipsum[2-4]}
\ParallelPar
\ParallelLText{\textbf{Main flow}}
\ParallelRText{Some itemize list here!}
\ParallelPar
\ParallelLText{\textbf{Postconditions}}
\ParallelRText{\lipsum[5-9]}
\end{Parallel}
\end{document}