Thanks for clarifying.
In that case, since you are using Twinmotion's terrain rather than an imported landscape, it currently isn't possible to have multiple terrain layers in one scene.
We currently don't have any plans to allow support for multiple Twinmotion terrain layers in a single scene, but I recommend suggesting your idea on our roadmap here so that our developers can review this request and see if it is a possible change we can make in the future.
Best,
Raghib