Re: Duplicated reharsal mark with combined two parts, one part has an acciaccatura, another part does not have an acciaccatura.

[Mark Polesky]

You could add a spacer acciaccatura to every voice
(by the way, "\clef treble" and "\key c \major" are 
defaults, so I removed them):

parttwo = \relative c' {
  \clef treble
  \time 2/4
  \key c \major
  \mark \default
  c''2 |
  \mark \default
  \acciaccatura {s8} c,2 |
  \bar "|."
}

but if you have a lot of voices, that's burdensome.
A better solution would be to define a global music 
block that contains everything that is the same for 
both parts:

global = {
  \time 2/4
  \mark \default
  s2 |
  \mark \default
  %% spacer acciaccatura aligns mark to barline:
  \acciaccatura s8 s2*2 |
  \bar "|."
}

Then you can "funnel two music expressions into one 
context". For individual parts, you would use:
  << \global \partone >>

And for the score, you would use:
  <<
    { \global }
    { \partcombine \partone \parttwo }
  >>

For an explanation, see:
http://lilypond.org/doc/v2.12/Documentation/user/lilypond/Creating-contexts#index-_005ccontext

See the attached files. Also, if you're still using 
2.11.65, you should upgrade to 2.12.1. 

- Mark

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