Heat your iron up to cotton setting (3 spots), or a lower setting if you have used polyester thread. Most quilters recommend a dry iron for this i.e. no steam.
Put your pair of squares on the ironing board and then place your iron on the seam and press down on it for a few seconds without sliding it around.
Cut along the marked line using scissors or your rotary cutter & ruler, this will leave 1/4" seam allowance between the stitched line and the cut edge.
Put your pairs of triangles back on the ironing board with the darker fabric on top. Now open the seam of the first pair (like a book) by folding the dark fabric over the seam and press it back in place, again try not to slide your iron about more than necessary as this can distort the fabric. You want the fold of the fabric to be as close as possible to the stitches. Repeat for the second pair and you will have two squares each made of two fabrics.
You can now trim your block to be absolutely square if required.