midi, channel

[Stefan Thomas]

Dear community,
is it possible, to assign different lilypond-instruments to different midi-channels?
I assume this done automatically, if one creates a percussion staff, but what happens with others?
I've openend the midi-file, produced with the below quoted snippet, with rosegarden and the two staves seem to be one instrument. If I change the Pan for one staff, it will also be changed for the 2nd.
How can I change it?

\version "2.16.0"
instI = {
  \set Staff.midiInstrument = #"voice oohs"
  c''1 R1*5
}
instIIII = {
  \set Staff.midiInstrument = #"voice oohs"
  R1 b'1 R1*4
}
\score {
  <<
  \new Staff \instI
  \new Staff \instII
>>
\layout { }
\midi { }
}