@igm,
They only need to exist in mechanical contact through spacetime.
if you describe spacetime in an ensemble through a geometric representation, you have several axis, the axis of space, one, two, or three dimensions, plus the dimension of time, yet another axis. Movement along these axis show the mechanical contact both in space as in time. They are not juxtaposed. You can say they coexist if you extend one dimension above the spacetime dimension and you observed all together in sequence in an ensemble, but remember even if in an ensemble they do not occupy the same spaces in the representation. They are along a group of axis.