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The diagonal cuts are made in different directions.
You can cut the two squares into triangles and sew them back together (the A to A and B to B).
Or you can sew the two squares together in the usual method of making HST blocks
first square, right side up, yellow, orange purple
Second square, right side down, purple, orange, yellow.
You have to make sure the middle seam, between the orange and yellow, are nested so they match up nicely when you sew and cut them apart.
You use the same method for the orange and blue HST blocks (background squares) - just check your orientation when either cutting the diagonals, or placing two squares together.