Imagine two jigsaw puzzles, with pieces matching in size.
One of them is slightly smaller than the other, having one fewer row and column.
They show scenes related in some way. For the sake of example, they show the same scene and viewpoint but with two different events occurring, perhaps at different
times of day.
The larger puzzle has a few judicious larger tiles, which are double the width and height of a standard tile - the sides of these have two knob/socket connections, so they can interface with standard tiles in the normal manner.
With the puzzles both individually assembled, place a dot of glue in the centre of each double-tile of the larger puzzle, then place the smaller puzzle centred on top, so it is offset by half a tile in both axes.
Allow the glue to dry, then break the prepared puzzle down into its component parts.
Now you have a stacked jigsaw, where you must assemble the lower puzzle first to complete the upper one.