Do I have to abandon partcombine and use the
<< {} \\ {} >>
construct to obtain what I want?
You can insert a \partcombineApart in the beginning of lh_one and restore the default behaviour m25 with a \partcombineAutomatic, when there are no more rests.
lh_one = \relative f {
\global
\clef bass
\partcombineApart % <- here
r4 c'2 c4 | % m1
...
r4 bf2 bf4 | % m24
\partcombineAutomatic % <- and here
c,4 g'2 bf4 | % m25
...
--
Gilles
