Undertie in lyrics to show no breath

[mancunius]

Hello. In the following example I want the undertie to be placed between the 2 
“O sacrum convivium” lines of lyrics, to indicate that no breath should be 
taken by the singers - as per the normal choral convention. (cf. the 
commented-out \breathe command in the music itself.) As it currently compiles, 
the undertie is placed beneath the word “O” of the second phrase. How do I 
adjust it so that it becomes a link between the 2 phrases, not part of one of 
them? (I’ve attached a screenshot of what I would like to achieve)

Thanks,

mancunius

\version "2.20.0"
\language "english"

global = 
        {
        \key d \major 
        }

soprano=\relative c'{
        r2 r4 fs
        b4 b8 b d2 ~
        d4 cs cs %\breathe
        d ~
        d8 cs b4 b as
        b2. fs4
        fs2 r2
}

sopranotext=\lyricmode{
O sa -- crum con -- ví -- vi -- um, \markup\undertie
O __ sa -- _ crum con -- ví -- vi -- um
}

\score {
        {
        \new Staff <<
        \global
        \new Voice="v1"
                {
                \clef "treble"
                \soprano
                }
        \new Lyrics \lyricsto "v1" {\sopranotext}
        >>
        }
}