>> A binary planet system might not be impossible...<<

That would be the easiest way to transfer between planets.

>> and some kinds of layered chemistry might be possible.

Think like deep-sea squid coming up to the surface every night...but on Venus or Jupiter that might involve a change in environmental composition as well as environmental pressure.<<

Yeah, I thought of that. Different gases have different weights. Here they all mix together, but there are a few places where smog gathers due to geologic formations. Another planet might conceivably have more variations.