Loc: Remember the Alamo - Texas
I think I first looked at this thread on the second day or so after you posted it. It was after AYCE / Paul at Thru-Hiker.com had left his comments.
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.
I looked for the low temperature you were trying for but don't think you posted one.
I have made a number of quilts and my share of different kinds of sleeping bags. I have tried about every kind of insulation that is easy to buy and one that is not easy to buy. I have some of each of the Primaloft insulation types that AYCE / Paul - Thru-Hiker sells except the ones he had pre-quilted with Momentum 90. AYCE / Paul had told me he was getting the Momentum / Primaloft pre quilted and I thought that was one of the best ideas I had heard in a long time.
I have a bunch of the Primaloft One 1.8 factory pre-quilted insulation and have used it for several insulated things. Primaloft One insulation is very good but if it isn't pre-quilted to something it is just not worth the extra trouble for me to use it.
I would not use Primaloft insulation for something as large as you have planned. Thru-Hiker suggested you use one of the Climashield insulations as they would not need as much (tying / quilting together). I expect that Paul at Thru-Hiker knows more about fabric and insulation than anyone. He has helped a lot of folks with material and insulation questions. He and I used to talk a lot about making gear and what material was best for a give item. I was always after the lightest possible weight for a given item so we did not always agree on the material. I have to say that I don't think he was ever wrong from the "normal" backpackers point of view.
For a quilt as large as you want to make, Primaloft One if it isn't pre-quilted is just to much trouble to use.
I would use Climashield XP in the 5 ounce per sq yard version.