a little warning against this concept:
I used a light sleeping bag for decades with this concept, much lighter insulation on bottom of bag, with the goal to reduce bag weight. It was however rather narrow (to maximize warming by having less dead air) but even as a slight fellow, I found in colder temps I would feel cold on my back when I turned in the night, because the whole bag would turn with me, exposing my back with hardly any insulation to the cold air when I was on my side.
It probably would have been alright if the bag had more real estate, more volume, allowing more turning to side to side easier without rotating the whole bag with you.
The quilit system with those two strap thingees does a pretty good job of keeping the quilt over you and fairly easy to keep it tucked up against you enough on both sides.
Good to know, thanks! Yeah, I'm a side sleeper anyway.
When I toured in flatland on a long-wheelbase recumbent with a B.O.B. trailer with a huge rubbermaid container in it, and rear panniers, I had huge amounts of space (don't even ask me the weight of the rig, it would have been impossible in hills, and even mild hills, I needed lower gearing than was available at the time, as I couldn't stand on the pedals). But anyway...
So much space I could toss the sleeping bag in loosely to dry out any sleep moisture during the day, the underside of the rubbermaid handles had vent holes shielded from any rain. I carried two lightweight down sleeping bags, rectangular, they were cheap on clearance, and nice to zip together for camping with my gal. But on that tour, I usually used one, and if it got cold, put one inside the other, plenty of space for it and also reduced empty air space, was very warm. On a different tour I brought one bag and a rectangular black fleece bag, nice for summer nights, but also, one night down at the lakeshore north of Chicago, I stealth-camped with no tent, on top of one of the huge flat rocks or broken concrete, can't remember, and the fleece over the down bag on top and the thermarest underneath, provided the right warmth, armored the down bag nylon taffeta and air mattress from abrasion, and provided black, non-reflective stealth.
These days I ride a 20" folder with full panniers, I'll never have the luxury of that much space again. I'll need to carry a single mummy bag that is more efficient in volume versus warmth.