Floating SliverAppBar elevation only when fully scrolled away

Question

All I want is to have 2 Appbars on top of each other. The top one is pinned and the one below is floating so it can be scrolled under the first. I always want to show a shadow underneath this "combined" bar.

So like this:

Appbar demo

But when the 2nd sliverappbar is visible the first one shouldn't have this elevation effect to divide them.

Tried to play around with the Sliver-/Appbar properties, NestedScrollView etc.