I need to do this at runtime so I cannot use this solution

I created my own script : when I load a terrain with a different resolution from these neighbors collects the terrain data and interpolates.

In recent versions of Unity Editor, seamlessness requires all terrains to have the same heightmap resolution.
When regenerating with high resolution, this requirement was violated and terrains lost their neighbors.
To fix this, select RWT Container / Utils / Update Neighbors.

I am currently creating an outdoor environment for an airplane simulator and I need to have France.
First, I made a generation of France (800x800km) in low resolution with tiles of 100x100km because the plane is 10 km above sea level.
Then, I generated a higher resolution terrain (100x100km) with 10x10km tiles for each airport I wanted to go to (for the moment Paris Toulouse and Lyon)
To finish I replace the tile of France with a terrain in high resolution.

Overall it looks good but the junction between these two terrains is not continuous. is there a way to fix that or a better approach to what i'm trying to accomplish.

Thank You !