A word for 'to exist in the same place as something else'

Solution 1:

Coincide works well. It's what I would use:

The graphs of f(x) and g(x) coincide between x=2 and x=6.

Dictionary.com's first definition is:

to occupy the same place in space, the same point or period in time, or the same relative position: The centers of concentric circles coincide. Our vacations coincided this year.

Solution 2:

Another word that would work well for two functions in a graph that meet would be intersect.

A word that would mean to exist in the same place as something else (geographically), would be colocated.

Solution 3:

You might also say there were conterminous - 'coincident in their boundaries; exactly co-extensive' (OED).