In the early 2000s I read a good sci-fi fantasy book about two binary planets that shared the same atmosphere at one point. The inhabitants used hot air balloons/gondolas to travel from one planet to the other.
What was the book called?
I've been searching on google for hours with no luck. However I can say its not the Flight of the Dragonfly/Rocheworld book series. Additionally, I suspect this book was used as inspiration for Lexx, season 3 "Fire and Water."