Simultaneous drum voices not playing in MIDI.

[Alexander Wallace]

Hi All the following looks good in PDF but produces no midi output. Can you 
help me guess what I'm doing wrong?

Thanks!

\version "2.16.2"

t = \drummode { \times 2/3 { sn4 sn sn } }
tp = \drummode { \times 2/3 { bd4 bd bd } }

unoM = \drummode {
  \time 2/4
  \t | \t | \t | sn4 sn | sn sn | \t | sn sn | \t |
  sn sn | \t | sn sn | sn sn8 sn | \t | sn8 sn sn4 | sn sn | \t | sn r \bar "|."
}
unoP = \drummode {
  \time 2/4
  bd4 bd | bd bd | bd bd | \tp  | \tp | bd bd | \tp | bd bd |
  \tp | bd bd | \tp | bd bd | bd bd | bd bd8 bd | \tp | bd4 bd | bd r |
}
uno = \drums { << \unoM \\ \unoP  >> }

\score { \uno \header { piece="Ejercicio 1" } \layout { } \midi { } }