It´s possible to sneak and fly at the same time in creative mode using touch input. Using a keyboard, sneak+fly works fine, this is just about touch. Currently, sneak+fly is not implemented in touch controls. The steps below are a bug that creates "sneak+hover if you hold the screen"

Steps to reproduce:
-Play in creative
-double tap the sneak button to start sneaking
-double tap the jump button to try to start flying
-then immediately press and hold the fly up button (it appears briefly, so be quick)
Now the Player is Floating but he can´t fly up only down.
if you want to fly up press the home button of your device and reentering MCPE (with out closing the app) or open the option menu. Now the Player is still sneaking and he can fly up and down.

Tested device:
Samsung Galaxy Note 3
Android 5.0
and
Samsung Galaxy Tab 3
Android 4.4.2

What another person thinks (i think he is right):

If you try to fly (by double tapping the jump button) when already sneaking, flying and sneaking are very briefly enabled at the same time then flying is disabled when we expected flying would never be enabled in the first place.

A side effect of that bug is that if you fly while sneaking and if you press and hold fly up before flying is disabled then disabled flying does not happen until you release fly up and while you hold fly up, your Player can sneak and fly at the same time (moving at sneaking speed in the air without being able to increase elevation). When you release fly up your player falls (at sneaking speed) and holding fly up will float the player at a lower elevation. When the player release fly up and lands on a block, the "temporary" flying is disabled.