It reminds me of Land and Overland by Bob Shaw: "The setting for the trilogy is a pair of planets, Land and Overland, which orbit about a common centre of grav

It reminds me of Land and Overland by Bob Shaw: "The setting for the trilogy is a pair of planets, Land and Overland, which orbit about a common centre of gravity, close enough to each other that they share a common atmosphere." They don't share a common atmosphere, but I guess the delta required for interplanetary transfer is really low.