I am not sure, I think the reason that they are asking for the two layers of fabric is because they are using the thicker type embroidery thread that they sell. It is a twisted cord like thread and you use one strand. But, I like tartan, use the regular thread and a good embroidery hoop and I also do not pull up tight on my stitches.
I personally think the extra layer of fabric would make quilting a pill. But, I suppose that is my own opinion.