partcombine

[Tim Litwiller]

http://lilypond.org/doc/v2.11/Documentation/user/lilypond/Vocal-ensembles#Vocal-ensembles

on this page
section


       D.4.2 SATB vocal score and automatic piano reduction

if you look at the piano reduction part it gets it right.  or at least 
in this sample.

so if I could just make it do the same thing with aikenheads and the 
words in the same staff I'd be pretty close.