Combining voices in American Hymns

[David F .]

Is there a way to combine two voices and print both stems (up and down) when 
the voices share a note?  \partcombine does not appear to do this by default.

American SATB hymns are typically engraved with the soprano and alto voices 
combined and the tenor and bass voices combined.  If a note in the soprano 
voice has the same duration as the note in the alto voice, then the notes for 
soprano and alto will share a stem.  If the durations are different, then there 
is no sharing.  And if the notes are the same duration and the same pitch, then 
the note with have both an up stem and a down stem.

For example, on this web page, about 2/3rds of the way down the page, you can 
see several hundred(!) scanned versions of Amazing Grace:

http://www.hymnary.org/text/amazing_grace_how_sweet_the_sound

The Lilypond docs discuss automatic part combining here:

http://lilypond.org/doc/v2.18/Documentation/notation/multiple-voices#automatic-part-combining

But I don’t see anything that would make \partcombine print two stems when 
voices share a note.

I’d appreciate any help.

David