Not the yarn weight, actual double knitting. Where you have a double thick fabric. Like in the Hoover baby blanket.
You can actually do smaller projects to get the feel of working any of the variations. A potholder, a doll blanket, or a scarf would all be good. I'm actually working on about a half size blanket for a kitty mat. Also a potholder/dishrag/dishtowel. I think I'll do a potholder in all three versions. I can never find one when I need one, so they won't go to waste and I can use some scrap yarns that I always have around. I also think a scarf would be nice for people who live in areas of very cold winter weather. I'd probably suggest a short one where you sew the two ends together so it slips over the head to avoid dangly ends that will likely unwrap the scarf from the neck. I forget if these types have an official name, but I'd rather wear one like that rather than a long one.