The Lilypond snippet
http://lilypond.org/doc/v2.19/Documentation/snippets/expressive-marks#expressive-marks-double-glissando
shows how to get a double glissando. It add adds a second voice to draw
the glissando lines with hidden notes:
\relative c {
\clef bass
<<
{
% new voice ( = \voiceOne), hidden
\hideNotes
% attach glissando to note heads
e2\glissando g
}
\\
{
% original voice with chords rearranged so that
% glissando is attached to a & c
<e a,>2\glissando <g c,>
}
>>
}
But the lower part designated with "original voice ..." is sufficient to
get the double glissando. I wanted just one glissando line connecting
the chords.
I have tested this with Lilypond 2.18.2 and 2.19.81. I wanted to check
this with older versions. But unfortunately the download site
(http://download.linuxaudio.org/lilypond/binaries) is down. Or the link
at http://lilypond.org/old-downloads.de.html is broken.