Double Pitched Trill

[Tiresia Giuno]

Hi all,

is there a way to get a double pitched Trill in LilyPond?
I tried the following code, but it displays only one note (the first
one) of the notes in parentheses

        \version "2.12.3"
        \include "english.ly"
        \relative c' { \pitchedTrill <f g>2 \startTrillSpan <gf af>
        ef\stopTrillSpan }

BTW, I don't understand why the LilyPond Music Glossary asserts that a
Double Trill usually occurs in the distance of a third
"A simultaneous trill on two notes, usually in the distance of a third."
http://lilypond.org/doc/v2.12/Documentation/user/music-glossary/double-trill