I did a 2-piece carve like that, what I did is I set the finishing pass slightly higher than I wanted, and let that complete. After carefully measuring how much more was needed, I lowered my Z Zero by that much, and re-ran the finishing pass.
As I found out with a second attempt, sometimes as the top layer is removed, it will want to warp, and can do so with enough force to move even though it’s attached to the second layer. In my case, the center of the piece rose upwards, and so what should have been land turned into water.