\new Staff <<
  \set Staff.soloText = "girl"
  \set Staff.soloIIText = "boy"
  \set Staff.aDueText = "together"
  \partCombine
    \relative c'' {
      g4 g r r
      a2 g
    }
    \relative c'' {
      r4 r a( b)
      a2 g
    }
>>