Variable number of voices on one staff

[Jordan Eldredge]

I am typesetting some music which includes a keyboard part. The right hand of the keyboard is predominantly one voice, but will occasionally split into two voices. I am currently using two voices where the first voice takes all the music that is only once voice, and the top voice when it splits. The second voice uses /skip or s1 to include the lower notes only when the part splits. The problem with this is that when there is only once voice, the stems all point up instead of pointing up or down depending on where in the staff they are.

What I want: When there is only once voice, the stems should behave as such. In measures where there are two voices, the stems should be dependent on voice.

My example code:

\version "2.10.25"

voiceOne = \relative c'' {r4 d4 c <d, bes'> | <c a'> <bes g'> <a d> <c g'> | <d fis>2 g}
voiceTwo = \relative c'' {s1 | s1 | s2 d,4 bes}

<<{\voiceOne} \\ {\voiceTwo}>>

Feel free to edit my example at my wiki (beware to slow edit times):

http://classicalcode.com/wiki/index.php/Two_voice_problem

The example is from:
http://classicalcode.com/wiki/index.php/Dido_and_Aeneas


Thanks,
-Jordan Eldredge