The answer has already been given, namely to use the method shown in
the second example of
http://lilypond.org/doc/v2.12/Documentation/user/lilypond/Common-not
ation-for-percussion#Percussion-staves
The trick is to use the standard > construct for
polophony. If you use version 2.12 or earlier, you also have to
explicitly instantiate two TabVoice contexts (just replace DrumVoice by
TabVoice in the example) called "1" and "2" (these names are hard coded
and cannot be changed). If you use version 2.13, there's no need to
explicitly instantiate these TabVoice contexts, LilyPond will
automatically do the right thing when it sees a > construct.
Not quite.... I tested this, and with the sampled I sent in my original mail
that worked fined.
Problem is, I need this construct in more locations, but it works ok only
the first time it occures :-S