How close could two earth sized celestial bodies get in their nearest approach either as planets or moons around a larger planet?
Close enough for atmospheric interaction in a stable orbit at nearest approach, or for non-space-fairing species to somehow make the jump without cosmological assistance (aka asteroid impact)?
It seems just from using the roche limit of an earth to earth sized bodies any approach under ~20,000 km would be unstable in the long term, which is orders of magnitude beyond a normal atmosphere size.