Can you knit both ways?
Double knitting allows you to create two layers of fabric at the same time, with only one pair of needles. The right sides of each layer face out, and the wrong sides face each other: stockinette stitch on both sides!
Does Double Knitting use more yarn?
Since double knitting requires you to have two layers of fabric this naturally means that the garment will be twice as thick which as you guessed means twice as much yarn will be needed.