Re: <<aes,2. \\ {s4 ees2} \\ {s2 bes4}>>

[Gianmaria Lari]

[...] 

Anyway, the output from the original code could possibly be improved by
instantiating the voices explicitly instead of using \\:

\version "2.19.82"
\new Staff {
  \time 3/4 \clef bass \key aes \major
  <<
    { aes,2. }
    \new Voice { \voiceTwo s4 ees2 }
    \new Voice {
      \voiceOne \hideNotes aes,4^(\unHideNotes s4 \voiceTwo bes4)
    }
  >>
}

You're right! Thank you Heikki!

g.