I believe you are talking of Ben Harris'
Superflip Change, as this uses a similar technique to the one you described. As far as I am aware, the Ben Harris change does not turn the cards over end-for-end, but rather flips them to the right; kind of like a book. The double bows downwards and is released firstly off the thumb, following by the middle finger -- however, it could be mistaken that they release at the "same time". This change also uses the index as a primary finger for the change's succession.
The Hiro Sakai
Duck Change, however, uses a completely different technique. Although the cards flip in the same way as does the
Superflip Change, this double bows upwards and is sprung off the middle finger before the thumb. This ensures that the double moves forward as the change takes place -- unlike the Ben Harris change, that does not more backwards OR forwards.
With that said, I unfortunately have no decent tips for you, I just wanted to make it clear for credit purposes that the two changes are not identical.
Take care,
Jordan