I have made lots of double slice quilts. I cut my own layer cakes, 10" squares. Need 40. Cut 3-1/2" pieces off each block.
That leaves 6-1/2" pieces. Turn over the 3-1/2" pieces, sew to the 6-1/2" pieces, press. Cut them in half. Mix up and sew back together, press. Sew with smaller squares at top, next to it sew with smaller piece at bottom. Make blocks like this. Next block turn blocks side to side.
You must have watched a different tutorial. Jenny Doan has tutorials on youtube.com. She will tell you and show you how to make her double slice.