markup question: combineParts

[Harry Plantinga]

I'm a lilypond newbie and I have a markup question that I can't figure out 
from the documentation. 

I'd like to combine two vocal parts, say Soprano and Alto, on a single staff, 
but the combineParts function only puts one stem on a note when there is 
a collision. I'd like to have both up and down stems. 

Is there a way this can be done automatically?

Harry