RE: partcombine, but including rests in quiet voices?

[Trevor Daniels]

> In this case, temporary voices won't work, 
> because then I no longer have both 
> instruments separately. 

If you already have the full parts for both
instruments separately then I think the approach
using two voices is still the best, as follows
below:
> 
> The other thing that I have tried is simply 
> putting both instruments on the 
> same staff as different voices. This, however, 
> has other problems:
> 1) notes of the same length are not combined into 
> a chord, 

No, but doesn't this make it easier to tell when
the two instruments cross over, for example?

> 2) stem directions have to be fixed with 
> \voiceOne / \voiceTwo, 

Defining a couple of variables makes this easier

> 3) When both instruments have a rest, two rests 
> are printed in strange 
> locations...

This can be fixed, as follows.  You have to tag
sections where the horns are in unison and where
they are separate in each part, made easier by
defining a couple of variables, in order to get
the rests right.

\version "2.11.34"

One = {\tag #'One {\voiceOne}}
Two = {\tag #'Two {\voiceTwo}}
Both = \oneVoice

HornOne = \relative c'' {
  % The entire part for Horn 1
  \Both R1 |  % Section in unison
  \One  R1 | b2.\f a4 | gis4  % Separate
  \Both fis4 e2 |  % In unison again
}

HornTwo = \relative c'' {
  % The entire part for Horn 2
  \Both R1 |
  \Two  a2.\f gis4 | fis2. fis4 | fis
  \Both fis4 e2 |
}


%  Print combined staff
\score { 
  \new Staff {
    \key e \major
    \time 4/4
    <<
      \HornOne
    \\
      \HornTwo
    >>
  }
}

% Print on separate staves
\score {
  <<
    \new Staff {
      \key e \major
      \time 4/4
      \removeWithTag #'One
      \HornOne
    }
    \new Staff {
      \key e \major
      \time 4/4
      \removeWithTag #'Two
      \HornTwo
    }
  >>
}