I have been using my JRB No Sniveller since spring of last year when I got it; I love It! Excellent craftsmanship and customer support from the Jack's, spot on temp rating IMO, made in the USA, super versitile quilt/poncho.
In the summer here in the cascades, I use my JRB No Sniveller, Ti Goat Ptarmigan bivy, BMW Torsolite, shorts, BPL beartooth hoody with great success. With lows that reach into the low 30's, this system is plenty warm for my blood.
As far as boosting the rating by adding insulation, my winter system will consist of a Montbell Alpine Light Jacket, JRB hood, BPL beartooth hoody, BPl merino tights, Montbell Thermawrap pants, and a pair of ID hot socks for the feet. I can push this into the teens and easily further;YMMV
With the double bag/quilt idea, you really have to make sure that the outer quilt is larger, so you don't compromise the loft. Maybe look into adding 1-2 oz's of over stuff to your quilt, or bumping up your insulation pieces=) To my crazy mind, the 2 bags/quilts seems super redundant; Not to mention it's not as verstile as a layering system that's been discussed. If you don't forget about the thread, let us know what you decided on and why.
I'd give the Jacks a call or shoot them an email to really dial in your system.