documentclass{article}

usepackage[series={A},noledgroup,nofamiliar]{reledmac}
usepackage{reledpar}
    setstanzaindents{12,0}
    setcounter{stanzaindentsrepetition}{1}

begin{document}

begin{pages}
begin{Leftside}
beginnumbering
begin{astanza}

First line text. edtext{An interesting&
Phrase}{Afootnote{A comment on two lines.}}. Second line text.&

end{astanza}
endnumbering
end{Leftside}

begin{Rightside}
beginnumbering
pstart

Right-hand side...

pend
endnumbering
end{Rightside}
end{pages}
Pages

end{document}

! Undefined control sequence. <argument> no@expands An interesting&Phrase l.33 The control sequence at the end of the top line of your error message was never def'ed. If you have misspelled it (e.g., `hobx'), type `I' and the correct spelling (e.g., `Ihbox'). Otherwise just continue, and I'll forget about whatever was undefined.

documentclass{article}

usepackage[series={A},noledgroup,nofamiliar]{reledmac}
usepackage{reledpar}
    setstanzaindents{12,0}
    setcounter{stanzaindentsrepetition}{1}

begin{document}

begin{pages}
begin{Leftside}
beginnumbering
begin{astanza}

First line text. edtext{An interesting}{%
  lemma{An interesting Phrase}%
  xxref{begin}{end}%
  Afootnote{A comment on two lines.}%
}edlabel{begin}&
edlabel{end}Phrase. Second line text.&

end{astanza}
endnumbering
end{Leftside}

begin{Rightside}
beginnumbering
pstart

Right-hand side...Right-hand side...Right-hand side...edtext{Right-hand side...Right-hand side...Right-hand side}{Afootnote{SNCF}}...Right-hand side...Right-hand side...Right-hand side...Right-hand side...

pend
endnumbering
end{Rightside}
end{pages}
Pages

end{document}