\documentclass{article}
\usepackage[series={A,B},noend,noeledsec]{reledmac}
\usepackage{reledpar}
%\setstanzaindents{1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0}
\setstanzaindents{1,0,0}
\setcounter{stanzaindentsrepetition}{6}
\parindent=0pt

\begin{document}
\begin{pages}
\beginnumbering
\begin{Leftside}
\stanza 1XXX XXX XXX. &
2XXX &
3XXX &
4XXX &
5XXX \&
%\begin{astanza} 1XXX XXX XXX. &
%2XXX &
%3XXX &
%4XXX &
%5XXX \& \end{astanza}
\endnumbering
\end{Leftside}

\begin{Rightside}
\beginnumbering
\stanza 1XXX XXX XXX. &
2XXX &
3XXX &
4XXX &
5XXX \&
%\begin{astanza} 1XXX XXX XXX. &
%2XXX &
%3XXX &
%4XXX &
%5XXX \& \end{astanza}
\endnumbering
\end{Rightside}
\end{pages}

\Pages

\end{document}