Temporarily combine two Voices on the same Staff

[H. S. Teoh]

I'm typesetting some complex polyphonic music, and have 3 Voices sharing
the same Staff. In the middle of the bar, the top two Voices share the
same rhythm, so I'd like to combine stems beams on them just for 2
beats, and have them be treated independently again afterwards. There is
a 3rd Voice on the same Staff with downward Beams, so without combining,
the top two Voices would have hard-to-read clashing beams.

Is there a way to do this without using \partcombine?  I realize that
it's probably possible to rewrite this with \partcombine, but it seems
to be overkill for just 2 beats.  I could probably also spell out the
combined chords directly in one of the Voices, but if at all possible
I'd like to represent it as separate Voices.


T

-- 
A linguistics professor was lecturing to his class one day. "In English," he 
said, "A double negative forms a positive. In some languages, though, such as 
Russian, a double negative is still a negative. However, there is no language 
wherein a double positive can form a negative." A voice from the back of the 
room piped up, "Yeah, yeah."