I did the same thing with two fleece blankets (the double layered with fringe everybody makes) a couple years ago, but I could not get them in the medium box when using one bag (did not know there was a large box) until I put each one in a separate "suck bag"(as the kids call it). Not only did I get two in the box but had room for other things.
When you are vacuuming the air out, you can put pressure on the bag as it goes down and get even more flatness out of them. I practically sat on one to make it fit!! lol