temporarily add extra staff - two times in a row?

[Johannes Waldmann]

Hi,

How could I get a extra staff temporarily,
and then a second time, and both at the same distance
to the staff that is running through?

I am starting from this:
http://lsr.di.unimi.it/LSR/Item?id=110
but it does not work two times in a row:

\score{
  \new StaffGroup {
    \new Staff \relative c'' {
      c1 | c << { c1 | d } \new Staff { c1 | b } >>
      % \repeat unfold 20 c1
      c1 | c << { c1 | d } \new Staff { c1 | b } >>
    }
  }
}

This compiles, but gives different distances.
If the two extras appear on different rows
(e.g., uncomment middle line to put some stuff in between)
then distances are equal again.

Each "\new Staff" opens a new context/engraver (?)
but this lives only on the "current line" ?
If that's somehow true - then how can I re-use the previous one?

Thanks - J.W.