# Mark where a line break should go IF the line needs breaking

I want to paragraph-break a text at a specific spot, but only if the text is too long for the line. In the MWE below, the first text fits the line width, but the second does not. Of course, I could just manually add a \par whenever the text turns out to be too long, but I would like LaTeX to spot this automatically [on a second pass, perhaps?] and break the line before the \Rbar symbol.

\documentclass[]{article}
\usepackage{xcolor}
\usepackage{gregoriotex}

\begin{document}%
\newcommand\versicle[1]{%
\textbf{\color{red}\Vbar. }{#1 }%
\textbf{\color{red}\Rbar. }{#2}}%
%
%This fits in one line
\versicle{Dóminus vobíscum.}{Et cum spíritu túo.}\par%
%
%This should line break at the \Rbar.
\versicle{Panem de cælo præstitísti eis, allelúia.}{Omne delectaméntum in se habéntem, allelúia.}%
\end{document}


The code produces this:

I'd like to get this:

You can test if the content width has exceeded \linewidth.

\documentclass[]{article}
\usepackage{xcolor}
\makeatletter
\newsavebox\testbox
\def\versicle#1#2{%
\savebox{\testbox}{\versicle@{red}{V.}~#1 \versicle@{red}{R.}~#2}
\versicle@{red}{V.}~#1
\ifdim\wd\testbox>\dimexpr\linewidth-\parindent\relax
\par
\fi
\versicle@{red}{R.}~#2
}
\def\versicle@#1#2{\textbf{\textcolor{#1}{#2}}}
\makeatother

\begin{document}%
%This fits in one line
\versicle{Dóminus vobíscum.}{Et cum spíritu túo.}\par%
%
%This should line break at the \Rbar.
\versicle{Panem de cælo præstitísti eis, allelúia.}{Omne delectaméntum in se habéntem, allelúia.}%
\end{document}