The standard situation is per structure. The need for insurance coverage is for the structure, they don’t insure the ground.
In this case, two certificates would be needed.
With respect to LOMA’s, they can be for a specific structure or a specific area. The key difference being is that the one using area would allow more than one structure to be considered out as well as future structures that fall within that footprint. However, future elevation certificates would need to the cite the LOMA and provide proof they are within that footprint. I have never had that situation arise (future).
Others here will respond with their own stories soon, I bet.