I have never slept with another person in a quilt or sleeping bag so I have no real idea how much warmer that might be. I did sleep with my wife many a night and know that two bodies, at times, can generate a lot of heat.
IMSHO, its' not effective because you get a nice big draft in between the two sleepers.
Now, I'd follow whatever Paul/Ayce has to say about working with quilted layers. He's "da man" as far as I am concerned, and has assisted me with some projects.
The quilted projects I've done have had the insulation quilted to the lining, and the main fabric free floating. This is how Paul designs many of the Thru Hiker items.