Newbie Q about stems and chords

[Thomas Ruedas]

Hi,
after finally having succeeded in compiling v.2.4.6 from the sources on my 
box, I have eagerly started playing with Lilypond, and I am quite 
impressed.
One of the things I have tried to find out concerns the notation of 
alternating stem orientation if one voice is distributed on both staffs 
of a piano score. I didn't find an example for my problem in the example 
collection or on Mutopia, so I'd be grateful for some help.
This works as expected (except for the odd extra bar at the end):

u = { \change Staff = upper  \stemDown }
l = { \change Staff = lower  \stemUp  }
i = \context Staff { \context Voice = "i" \voiceOne
\clef bass \key bes \major \time 2/4
\relative { \u bes,16 \l bes \u bes \l bes \u bes \l bes \u bes \l bes }}
ii = \context Staff { \context Voice = "ii" \voiceTwo
{ \clef bass \key bes \major \time 2/4  } s1 }
\score { \context PianoStaff <<
        \context Staff = "upper" << \i  >>
        \context Staff = "lower" << \ii >>  >>
    \layout { \context { \Score
            \override SpacingSpanner #'spacing-increment = #3 } }

However, if I want to set a chord instead of a single note, I get errors; 
a line like this
\relative { \u { <bes,16 f> } \l bes \u bes \l { <bes bes,> } \u { <bes 
f> } \l bes \u { <e fis> } \l bes }

results in Lilypond complaining:

error: syntax error, unexpected UNSIGNED, expecting DRUM_PITCH or 
NOTENAME_PITCH or '>':
\relative { \u { <bes,1
                       6 f> } \l bes \u bes \l { <bes bes,> } \u { <bes 
f> } \l bes \u { <e fis> } \l bes }

I have tried a couple of variations in placement etc., but the error 
remains in principle as shown here. Why does that happen, and what can I 
do? What is the orthodox way to typeset this?
Thanks for any help,
Thomas
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