midi for many pieces

[Daniel Hernandez]

Hello I working with a composition with many pieces. I structure all they
in a book enviroment like this:

--- file book.ly ----------
: 
: Flute = \relative c'' {
:   \key c \major
:   \time 4/4
:   a b c d f g a
: } flute = {
:   \set Staff.instrument = "Flute "
:   \set Staff.midiInstrument = "flute"
:   \clef treble
:   \context Staff << \Flute >>
: }
: 
: Violin = \relative c'' {
:   \key c \major
:   \time 4/4
:   g f e d c b a g
: } violin = {
:   \set Staff.instrument = "Violin "
:   \set Staff.midiInstrument = "violin"
:   \clef treble
:   \context Staff << \Violin >>
: }
: 
: \book {
:   \header {
:     title = "Book test"
:     composer = "My name"
: }
: 
:   \score {
:     \header { piece = "Piece 1" }
:     <<
:       \context Staff = flute \flute
:     >>
:     \layout { }
:     \midi { \tempo 4=60 }
:   }
: 
:   \score {
:     \header { piece = "Pice 2" }
:     <<
:       \context Staff = violin \violin
:     >>
:     \layout { }
:     \midi { \tempo 4=60 }
:   }
: }
:
---------------------------

But when I compile the *.ly file it generate only the midi for the second 
score. For the compilation comments above I think that it overwrite the 
first score.

address@hidden lilypond book.ly
GNU LilyPond 2.4.5
Processing `book.ly'
Analizando...
Interpreting music... [2]
Preprocessing graphical objects...
Calculating line breaks...
Interpreting music...
Salida MIDI a `book.midi'...
Track...
Interpreting music... [3]
Preprocessing graphical objects...
Calculating line breaks...
Interpreting music...
Salida MIDI a `book.midi'...
Track...
Layout output to `book.tex'...
Converting to `book.dvi'...
Converting to `book.ps'...
Converting to `book.pdf'...

I want to get one midi file, with the two consecutive scores, or two midi 
files, one for each score. Do you know how can I do it?

Thanks
Daniel Hernandez