How to load/make hybrid-resolution terrain tiles?

Hi, I have created global terrain tiles with low resolution which generates 10 levels(0-9), now I need to load DEM of a small region with higher resolution, I tiled the region’s DEM with Cesium Terrain Builder, and I copied the higher level data folder into the global terrain data folder to make the level increase from 9 to 12, wishing it to be a hybrid resolution terrain tiles. However each time I load the terrain, the higher tiles form 10-12 levels won’t be loaded. I am sure the tiles are right handled and only thing is terrain folder from 10-12 levels only contains the tiles generated by higher resolution DEM. Is there anything wrong for me to mix them up into one category to make it a hybrid-resolution terrain data?