This could probably be extended to more than two images. Don't know how far you can push it before the images look too contrived to be interesting.<p>In thinking how I would start, I imagined starting with only one kind of shape, except filling/flattening out the edges/corners. Then come up with a dual picture that depends on 2 arrangements of the pieces. Finally adjust the edges of the piece shapes without breaking the 2 selected arrangements but allowing all the other possible arrangements to collapse down.<p>It's like making a repeating/tiling shape where you start with a blank tiled plane then start drawing anywhere, all the repeated places also get drawn in, and you keep drawing/undoing until you like what you see. i.e. 'maintain the constraint from the beginning'. The tricky bit is in the drawing/painting. I imagine the same could be done with this jigsaw, blank the pieces, show both arranged blanks, and let someone paint on the pieces updating both as you go. When you like what you see, you're done.