Voices using comon stems

[Felipe Massia]

Hello,

I need to typeset a SATB score with S+A and T+B using common stems.
Here's what lilypond does (with SATB template):

  +--+   +
  |  |   |\
S o  o   o
A o  o   o
  |  |   |/
  +--+   +

But I want:

  +--+  +
  |  |  |\
S o  o  o
A o  o  o

I tried some combinations of voices and I did not suceed. When I use
stems up for Sop and Alto, I get something like...

  +--+  +   <-- double
  +--+  +\
  |  |  |\
S o  o  o
A o  o  o

It seems the case is that lily is handling voices separately. I think I
should use polyphony with <...> but I suspect there is a more structured
and maybe easier way, since it's a common feature. If I use <...> I'm
afraid I wont be able to tell the 4 voices in the MIDI result, but only
2 dual voices.

Is there a way?

thanks in advance,
-- 
Felipe Massia Pereira