how make "swappable" tile variations (common border)?

I have a situation where it would be useful to make several "seamless" tiles that could be "mixed and matched" -- placed side-by-side in random order, and would still be seamless. That is, all the tiles of a "set" are identical at their borders, yet in the interior, they are different. Imagine some pattern like cracked earth, where the cracks and splits that cross an edge have to be identical in all variations, but as you move away from the edge, the cracks would vary. So I was thinking maybe this could somehow be done using 2x2 tiling (make all 4 variations at one time, each 1/4 of the total image), and somehow pin the cracks as they cross borders, but let them gradually randomize in the interiors of each quadrant .. but now I am over my head, having not made a filter ever; I'm just hoping to modify some existing ones. Suggestions?

Totte, thank you -- that is what I needed -- the Profile Gradient that goes from white in interior to black at edges, used to control Blend Opacity, to switch between two sources, one of which is held constant (for a given variation). I have played with this to make a filter which holds the entire edges constant, letting one change a slider to control interior colors ("(IV) Interior Colors") to go with your other IV (Interior Variation) sliders. My example is set to more dramatic color tiles, to make the technique you used clearer.

... My version attached here, as it might help someone else understand Totte's technique.

cracked-stone vmulti tilesl Constant Border.ffxml