staff manipulation question

[Alex Jones]

I’m working with a score that has multiple staves that appear and disappear at 
various points.

I ran into a curious behavior:

I have two staves fro a while, the second has two notes that complete 
mid-measure so I encoded like this:

\score {
\new StaffGroup \relative c' {
\new Staff
\key f \major
\time 6/8
<<
{
R2.*2
e2.~ \pp
}
\new Staff \relative c'' {
\once \set Staff.explicitKeySignatureVisibility = #end-of-line-invisible
\set midiInstrument = #"violin"
\once \omit Staff.TimeSignature
\key d \minor

<g bes,>2.
<< {g~ g8} \\ {f2. ( _\markup{\italic dim.} e8) \pp} >> r8 r r4 r8
}
>>
\break

a8-. \p ^\markup{solo arco} [r16 g-. a8-.] c-. r r

}
}

However, when the second staff concluded, it didn’t disappear.  I worked out it 
was because the << >> region wasn’t aligned with a measure break.

So, I replaced with this:

<< {g~ g8 b\rest b\rest b4\rest b8\rest} \\ {f2. ( _\markup{\italic dim.} e8) 
\pp s s s4 s8} >>

however, this was quite clunky.  I had to create rest space in both note 
groups, and then I had to reposition the rests in the top note grouping to 
apply to both voices.  Aside from the staff not disappearing, the top solution 
works well.  

Is there a more streamlined way to do this?

Using lilypond 2.18.2

-akj