Back to the drawing board (Sib->Ly)! And question about voices

[Kirill]

Ok, gentlemen. Summer is over, back to the drawing board (for those who still
remember, the Sibelius->LyliPond plugin is in works).

The following question might seem a bit unusual, but I'll ask anyway.

Suppose I have a Staff, and two explicitly instantiated Voices in this Staff.
These voices are marked \voiceOne and \voiceTwo, which, in particular, controls
the stem directions. Both voices span for the whole duration of the music,
regardless of whether one of them is actually necessary or not. When there is no
music in one of the voices -- it is padded with hidden rests (like s1*1/2).

Now, the question. Is there a way to make LilyPond automatically choose the
correct stem directions in the first voice based on whether there is something
in the second? For example: imagine a bar where voice 1 has some music, voice 2
is not being used (but exists) and is padded with "s1*...". I want the stems and
everything to behave like there is no voice 2 in this bar. Is it possible?