There's also the dimension of time. Two such civilizations need to be both spatially and temporally proximal. One of the "explanations" for the Fermi Paradox is

There's also the dimension of time. Two such civilizations need to be both spatially and temporally proximal. One of the "explanations" for the Fermi Paradox is that humans are too far out from the temporal peak, i.e. too late or too early to the scene.