partcombine doesn't work with 'clashing notecolumns'

[arie-lily]

Hello,

I tried to put double voices on some staffs with \partcombine but the
second staff doesn't work. The interpreter complains as follows:

Interpreting music...[8][9]
Preprocessing elements...
Calculating column positions... warning: Too many clashing notecolumns. 
Ignoring them.
warning: Too many clashing notecolumns.  Ignoring them.
warning: Too many clashing notecolumns.  Ignoring them.


The .ly file is the given below. Is it possible to get a proper
partcombine, or can I just simply forget it?

Thanks,
arie

========================
\include "paper16.ly"
\version "1.6.2"

global = {
    \notes {
        \time 4/4
        \key f \major
    }
}


musicAltOne = \notes \relative c'' {
    \repeat unfold 6 r1
    bes8\p \repeat unfold 7 bes8
    \repeat unfold 8 a8 
}

musicAltTwo = \notes \relative c'' {
    \repeat unfold 6 r1
    g8\p \repeat unfold 7 g8
    \repeat unfold 8 f8
}

musicTenOne = \notes \relative c'' {
    r1 r1 r1 r1
    c8->^\p r8 r4 r2
    c8-> r8 r4 r2
    d8-\p r8 r4 r4 d8 r8
    c8 r8 r4 r4 c8 r8
}

musicTenTwo = \notes \relative c'' {
    r1 r1 r1 r1
    bes8 r8 r4 r2
    a8 r8 r4 r2
    bes8 r8 r4 r4 bes8 r8
    a8 r8 r4 r4 a8 r8
}

wordsAltFo = \lyrics {
    Po -- pid ti -- nom bi -- zjitj ma -- ti,
    swo -- joe do -- tsjkoe pe -- rej -- ma -- ti. 
}

wordsTenFo = \lyrics {
    Oj! Oj!
    doeb, doeb, doeb, doeb, doeb, doeb, 
}


\score {
    \context StaffGroup <
        \context Staff = staffAlt {
            \addlyrics
            \context Voice = voiceAlt {
                \global
                \partcombine Voice
                \context Thread = threadOne \musicAltOne
                \context Thread = threadTwo \musicAltTwo
            }
            \context Lyrics = wordsAltFo { \wordsAltFo }
        }

        \context Staff = staffTen {
            \addlyrics
            \context Voice = voiceTen {
                \global
                \partcombine Voice
                \context Thread = threadOne \musicTenOne
                \context Thread = threadTwo \musicTenTwo
            }
            \context Lyrics = wordsTenFo { \wordsTenFo }
        }
    >
}

\paper {
}