Knitting with two strands gives you thicker yarn, which will make the blanket a bit bigger but also thicker/stiffer if you don't adjust needle size.
If you want to keep the drape as in the photo, then when you work two strands you should also go up in needle size. The larger needles will increase the size of your blanket as well.
A swatch done with the two strands and larger needles will tell you how much larger the blanket will be just from these adjustments. If you want the blanket to be even larger then you would increase the number of cable repeats within the pattern. You do not need to increase the number of border sts.