Before I upgraded to a real sleeping I used to double bag also-- I started with a +20F quilt and a +50F bag combined with capilene baselayer top, bottoms as well as down pants and jacket inside a double or single walled 4-season tent. With this I could go down to zero without too many issues, I wasn't exactly warm but I wasn't uncomfortable either.
So based on my experiences I would give the two +30F quilt/bag combo a try while car camping or somewhere with a safe retreat.
However, I would say that with a single, full length ridgerest at zeroF, I would be getting pretty cold from below. I think if the double bags works, then I would look at adding additional pads or investing in something like the Exped 7-- well worth the cost/weight in the winter.
As I said, I have since upgraded-- I found that for true mountaineering, above treeline with temps ranging anywhere from zero to -20F, that a quilt was just a poor choice-- far too drafy and combined with another bag, I wasn't really saving any weight or space over a quality zero degree down bag.
These days I carry the following:
-FA Down sweater
-FA Mountain Guide Hooded Jacket
-Capilene 3 bottoms
-FF Volant pants
I usually find the combined with a hat and various gloves that this system covers me for a wide range of temperatures (-15F to about +40F) and for everything from high aerobic climbing over 13K in Colorado, to sitting idle in base-camp.