grace note synchronization and multi-rests

[Werner LEMBERG]

Assume the following score

  <<
     \new Voice \relative c' {
       R1*2 |
       \acciaccatura { c8 } c1 | c1 }

     \new Voice \relative c' {
       R1*2 |
       \acciaccatura { s8 } R1*2 }
  >>

To synchronize the grace notes it is necessary to insert them into the
second voice also (as skips).  Extracting this second voice in a part
I want to get

       4
   |-------|


but I get

      2       2
   |-----| |-----|


due to the grace notes.  What must I do to have a 4-bar multi-rest
automatically?  I know that I can insert a kludge with \tag to provide
different versions for the part and the full score, but...


    Werner