I used Hobbs 80/20 and Polydown on a quilt. It came out very heavy and a bit too stiff, although I've only washed it once, so it might soften up over time. It doesn't feel that thick, just heavy.
On my last quilt I used Quilters Dream Puff which was such a pleasure to quilt, and the quilt seems so soft and warm. They claim it is warmer than down. That's what I would recommend instead of a double layer.