combine staffs by using a comment

[Stefan Kegel]

both GrandStaffs are equal, but in the second Staff there is a comment
inserted to \set staff.instrumentname"


\version "2.10.33.1" 
%...................................................first Staff without comment
\new GrandStaff
\relative 
{<<
\new Staff 
   \set Staff.instrumentName = "1. 2." 
  \relative c''
   {
     \clef treble
     \time 6/8
      {<< {c2.|}\\ {g2.|} >>}  
   }

\new Staff  
   \set Staff.instrumentName = "3. 4."    
  \relative c''
   {
      \clef treble
      \time 6/8
       {<< {e2.|}\\ {c,2.|} >>}  
   } >>}

%...................................................second Staff

\new GrandStaff
 \relative c''
{<<
\new Staff 
  %\set Staff.instrumentName = "1. 2."                     %comment inserted
  \relative c''
   {
     \clef treble
     \time 6/8
      {<< {c2.|}\\ {g2.|} >>}  
   }

\new Staff  
  %\set Staff.instrumentName = "3. 4."                     %comment inserted
  \relative c''
   {
     \clef treble
     \time 6/8
      {<< {e2.|}\\ {c,2.|} >>}  
   }>>}