I've often relied on using two bags as my winter set up since it gives me a little bit more flexibility in choosing what sleeping set up I want to bring from the early to mid season.
If someone is looking into doing a sleeping bag/quilt combo, I picked up Big Agnes's Lost Ranger 3N1 UL, which is an ultralight sidewinder sleeping bag that goes into a quilt that cinches to a sleeping bag. It's a pretty good design, but the zippers are kinda bad. Putting zippers on a quilt feels like it's adding unnecessary weight and complexity.