I'm not sure I understand exactly where the second slur supposed to go.
You say "above", but explicitly specify the slur to go below.
Which is to say, you write _( _) instead of just ( ).
In any case, one way to get two slurs is to use
phrasing slurs in addition to regular slurs.
http://lilypond.org/doc/v2.18/Documentation/notation/expressive-marks-as-curves#phrasing-slurs I adjusted your example to do this.
However, I don't understand why the phrasing slur
does not show up in the tab staff, only in the regular staff.
Maybe someone else can figure that out.
To get the phrasing slur to move up from where it normally sits,
you can use \shape.
http://lilypond.org/doc/v2.18/Documentation/notation/modifying-shapes You could specify the phrasing slur to be above,
either by using ^\( ^\), or by using \\phrasingSlurUp,.
However, this puts the slur above the entire staff,
and that is much further away from where (I think)
you want it, so we'll keep it as it is, below.
I also demonstrate use of tags, so that the modifications
for \shape only apply to the staff, not the tabstaff.
http://lilypond.org/doc/v2.18/Documentation/notation/different-editions-from-one-source#using-tags\version "2.18.2"
\header {title = "Simultaneous slurs in a tuplet?"}
global = {
\key c \major
\time 4/4
}
% I put your global definitions in parallel with the music,
% since if you start to add things like barlines and anything
% else that is not at the very beginning,
% you won't be able to use \global in sequence like you did.
flamencoGuitar = <<
\global
% I NEED TO ADD A SECOND SLUR ABOVE THE FIRST HERE
\relative c {
\tag #'RegularStaff {
\shape #'((0 . 0) (0 . 0) (0 . 0) (0 . 0)) Slur
\shape #'((0 . 3.8) (-0.2 . 4.8) (-0.2 . 5.4) (0 . 4.0)) PhrasingSlur
}
\tag #'TabbedStaff {
\shape #'((0 . 0) (0 . 0) (0 . 0) (0 . 0)) Slur
\shape #'((0 . -0.5) (0 . -0.5) (0 . -0.5) (0 . -0.5)) PhrasingSlur
}
\tuplet 3/2 << { <d g c e >8 \( _( <f a> _) \) <f a c e > } >>
}
>>
\score {
\new StaffGroup <<
\new Staff \tag #'RegularStaff { \clef "treble_8" \flamencoGuitar }
\new TabStaff \tag #'TabbedStaff \flamencoGuitar
>>
}
HTH,