Can I double up my yarn so I can knit it extra fast and large?
Doubling yarn won't necessarily knit faster, larger needles will knit faster.
Doubling the yarn AND larger needles will knit faster but also be a looser knit scarf because the stitches will be bigger.
The best (though not the quickest) answer - knit a swatch. That will show you what it will look/feel like, then you can change your needle size if you want.
Maybe that's what I will do. I will do extra large needles and then sew a piece of fleece on one side. Thanks. I really would like to get one done for Christmas.