overriding default tuplet number

[Bret Whissel]

I've got music in a triple meter, and I'd like the tuplet in the
following instance to read "2" instead of the default "4".  Anyone know
how I can achieve that?

\include "english.ly"
\score {
  \relative c'' {
    \time 12/8
    \key d \major
    bf4. \times 3/4 { a4 c } bf4. a
  }
}

(I'm using LilyPond 2.8 since I've tweaked the installation, but I'd be
willing to upgrade to 2.10 if necessary.)

Thanks,
Bret