another problem with partcombine

[Stefan Thomas]

Dear community,
I've got another problem with partcombine.
Is there a possibilitie to force lilypond to print  the following (whithin the partcombine-command) in tow different voices?
\version "2.12.2"

upper = \relative c' {
  r8 c e g e c
}
lower = \relative c' {
  c4 r2
}

all ={  \new Staff { \partcombine \upper \lower  } } 
\score { \all  }