I have been showing this technique to my customers for the past month or so. I cut my binding 1 3/8" for the main binding and 1 5/8" for the faux piping. I like to cut my bindings at 2 1/2" and once you sew these two together you get 2 1/2" binding. I just love the look of this binding, and if you are short your main binding fabric this helps stretch it so to speak.
Have fun with this all done by machine binding. No wonder they call it Magic!
Here is the link to see how 52 quilts dot com does theirs.