Add second set of lyrics part way through a movement

[Dirk Koopman]

I am trying to set a Victoria mass's Agnus Dei. Most of the words are 
the same, but there are two endings, basically "miserere nobis" and 
"dona nobis pacem".

textocantus=\lyricmode{
  A -- gnus De -- _ _ i,
  A -- gnus __ _
  De -- _ _ _ _ i,
  A -- gnus De -- _ _ _ _ _ _ _ i,
  qui tol -- lis pec -- ca -- ta mun -- _ _ _ _ _ _ di,
  qui tol -- lis pec -- ca -- ta mun -- _ _ di,
  pec -- ca -- ta mun -- _ _ _ _ _ _ _ _ di, __
  pec -- ca -- ta mun -- _ _ _ _ _ _ di,
}

tcm=\lyricmode {
  mi -- se -- re -- re no -- _ _ bis,
  mi -- se -- re -- re no -- _ _ _ _ _ _ bis,
  mi -- se -- re -- re no -- bis,
  mi -- se -- re -- re no -- _ _ _ _ _ bis,
  mi -- se -- re -- re no -- _ bis.
}

tcd=\lyricmode{
  do -- _ na no -- bis pa -- _ _ _ cem,
  do -- na no -- bis pa -- _ _ _ _ _ _ cem,
  do -- na no -- bis pa -- cem,
  do -- na no -- bis pa -- _ _ _ _ _ cem,
  do -- na no -- bis pa -- _ _ cem.
}

The first part would look something like this:

\score {
  \new ChoirStaff <<
    \new Staff = "v1" {
      \set Staff.instrumentName = "Cantus"
      \set Staff.shortInstrumentName = "C"
      \clef treble
      \new Voice = "v1" {
        \global
        \cantus
      }
    }
    \new Lyrics = "v1"

    \context Lyrics="v1" {
      \lyricsto "v1" {
        \textocantus

        %<<
        \tcm

        %\tcd
        %>>

      }
    }

...

The net result wants to look rather like this:

"... peccata mundi, miserere nobis."
"                   dona nobis pacem."

Any suggestions?

Dirk