Again: stemBoth problem

[胡海鹏 - Hu Haipeng]

Hello,
  I encounter a new problem with this function:

stemBoth = #(define-music-function (parser location m) (ly:music?)
#{ << \voiceOne $m \new Voice { \voiceTwo $m } >> \oneVoice #})

  When I attach the slur, it is copied too. Now I want a function to write things like this: the first half of the passage is played by player 1, and the other by 2, with an overlapping note where I need this function. I don't want to assign two separate voices with plenty of confusing spacer notes. Since I can't see, I don't know whether this gives two stems:

\relative c' {
  \stemUp c8(^"I" d e f g a b c | <d d>)(_"II" \stemDown c b a g f e d | c1) |
}

  If I add \stemDown in the chord to the second d, then Lilypond will give error and refuse to compile. Could you help me?

Haipeng