You'll be doubling the strands, but your blanket will get bigger faster. I'm not sure if you'll have to do half the stitches to get the same size or not. I suppose you could check the gauge of the pattern compared to the gauge you're getting on the doubled yarn and big needles and see? Sorry, I don't really know how to compute this. :??
I'm pretty sure that you won't use twice the yarn. I would go as far as I could and see if it's big enough, but that decisions up to you.