Hi,
glad to inform you that the newest version 2.4.3 presents new feature: one-finger zooming: double tap on map and drag your finger (up and down to zoom in and zoom out).
I agree with Tmurakami.
It is difficult to operate on smartphone with two finger. The best solution I have found is the one used by Google Maps for iphone. Tap and swipe. The zoom-in or out is continously during swipe.