Hiding specific staves/removing delimiters

[Bridge]

Hello, 

I have a slight problem with writing divisi strings on two staves. If I
write the parts on two independent staves and connect them with a square
delimiter, later on in the score when the strings are to play in unison, the
delimiter persists and I haven't found a way to remove it. I came up with a
somewhat inelegant solution to this problem, which is to simply write the
divisi sections on a grand staff with a switched out delimiter and making a
new hidden staff which contains the unison music. This works rather well,
except for the fact that the hidden staff shows up on the first page. Is
there any way to hide the staff absolutely until it is needed? If not, does
anybody know how to remove a delimiter mid-score?

Thanks in advance.

P.S.: If it wasn't obvious, this is what I mean:

\version "2.16.0"

\score {
        <<
                \new GrandStaff <<
                        \new Staff { s1 }
                        \new Staff { s1 }
                >>
                % Should be hidden
                \new Staff { s1 }
        >>      
}