combining simultaneous notes

[Arno Waschk]

dear list,

(sorry for cross posting, i am not sure where to address correctly...)

i have a problem which seemed trivial to me, but i cannot resolve 
according to the docs successfully:

\version = "2.19.22"
aa={a4 b4 a4 b4}
bb={a4 g a g}
\new Staff {
<<
  \aa
  \bb
>>
}
\new Staff{
a4 <g b> a <g b>
}


prints two lines. How can i make the first to look like the second.

\partcombine does not work for me in more complex situations.

Is there something like \mergeAllNoteHeadsPossible    ?


Thanks, Arno