Hmm, can't seem to get it anymore.

Edit: After messing around and getting about a 10% success rate in double-tapping with one thumb or both thumbs, I think I figured the surprisingly sensitive requirements. You can't tap with an area of your finger that's too small or big, but the speed of the double-tap is most important, so experiment with that to find it (hint: it's definately on the fast side). I can get it up every time now (hardy har).