span, range, extend over
亘 centers on spanning across space or time: extending over a range, covering a distance, or reaching from one point to another.
亘 is a pictograph of a boundary or line extending across, possibly representing a horizon or a measuring line. The modern form retains the idea of spanning.
The top line 一 and the bottom 旦 (sunrise) look like the sun spanning the horizon from dawn to dusk, covering the whole range of the day.
For コウ, imagine a long spanning bridge over a wide river, and a person calling out 'Go!' to start crossing: Go -> コウ, spanning the distance.