Can a habitable planet have two separate crusts, both of which can sustain life? I am wondering if through the planet's creation, over time through some form of mantle cooling/erosion combination, without any intentional artificial construction, a planet can have two crusts, with air in between.
The larger the distance between the two, the better. Ideally, the distance between the two crusts would be greater than 12 km, allowing for flight. I know the atmospheric conditions would be vastly different between the two crusts, and I'm wanting the bottom crust to have similar conditions atmospheric conditions to those on earth, even having weather patterns. This would lead to an insanely thin atmosphere around the top crust.
The only way I can see working is some form of massive cave system forming in one of the thickest crusts ever developed or some columns that remain from erosion that has removed an area to create a troposphere. For simplicity sake, I'd say we just assume the top crust has enough holes to allow in light to sustain life on the bottom crust.
I do not know how to get the area between the two crusts sufficiently lit, but that would help too. The implications on society, evolution, and life in general, would be extremely fun to explore, but I want to know if the world is even possible without some sort of magic intervention.