Would it be possible for a planet with two extreme altitudes (very low lowlands and very high mountain regions) to have two habitable zones separated by a layer of toxic air? Like an atmospheric sandwich with a breathable atmospheric layer in the lowlands, a non-breathable atmospheric layer in the middle, and a breathable atmospheric layer in the highlands.
Honestly - probably not. Atmosphere’s tend to mix a lot, with things at the bottom of the air blanket eventually making it’s way to the top and back down again. It’s part of having weather.
Also, the mixture of air that’s breathable has a general density which would almost certainly be different from a mixture of unbreathable, poisonous air. If the poisonous gases were more dense, they’d wind up at the bottom of the air blanket and fill your valleys and lowland. If they were less dense, they’d rise to the top of the atmosphere.Now that doesn’t mean it couldn’t happen, but it would probably not be a natural phenomenon. It could happen in an underground world where the poisonous gases lay in the low parts of tunnels and cut off some caves from others. But for a whole planet, it’d very likely have to be something unnatural. Pick the method - sci-fi force fields, magical waves, or whatever that separates the low-dwellers from the high-ups.
