Hi Glenn,
I tried using progressive in code:
RexTerrainEngineOptions opt;
opt.progressive() = true;
auto map = new Map{opt};
auto mapNode = new MapNode {map}
but I couldn't see any difference in the loading of the layers, meaning that if I load the layer when zoomed in, but then zoom out, the layer flickers in and out.
Also, the problem of loading 2 different layers one after another persists.
It looks to me like the first layer begins loading, and then when the second layer begins loading the first layer gets stuck until the second layer loads.
This leads to the globe looking like this for a couple of seconds:

In the previous version (2.9) the layers loaded one after another without a problem.
Eran Cohen