Two strings writing legato

[Stemby]

> I'm not top posting.

Hi,
I'm not sure that it is a bug.

I can't realize a two strings writing legato like this[1].

Here, a little example who doesn't work:
----
\version "2.10.33"

\paper{ ragged-right=##t }

melodyOne =  \relative a' {
        f16( [ a ] s16 a16)  s16( a16 [ s16 a16) ] s16 a16 [ s16 a16 ] | % First
quadruplet OK, second quadruplet NO
    }

melodyTwo =  \relative f' {
        d16 [ s16 a'16 ] s16 bes16 [ s16 a16 ] s16 g16( [ s16 f ] s16)  | % 
Third
quadruplet NO
    }

melody = << \melodyOne \\ \melodyTwo>>

% The score definition
\score {
        \context Staff << 
        { \clef treble \key f \major \time 3/4  \autoBeamOff \melody  }
    >>
        \layout { }
}
----

How can I obtain a correct quadruplet legato? Is it possible or it is a bug?

Note that this writing is quite frequent with string instruments: you could
include a correct example into the user manual.

Regards

Carlo


[1] http://img505.imageshack.us/img505/3504/dsc02238gy0.jpg