What combined temperature rating are you looking for?
To estimate the combined temp rating, read the following two threads of Richard Nisley's work:
[Note: I don't believe the following paragraph, but perhaps someone will point out an error.]
Reading the chart at the beginning of the first thread, it looks like a true 30 degree bag needs to have a clo of 6.5. Then you have to figure out the clo of the overbag/overquilt. If it's also rated at 30 degrees, or 6.5 clo, then the total will be 13 clo. Extrapolating the graph, that would make the rating about -30F. This seems ridiculously optimistic to me. It would be hard to seal leaks with an overquilt; I think it would have to be a pretty large overquilt.
Some people use a combination approach for New England winter camping with a large synthetic outer bag and a down inner bag. Compressing loft can be a problem. Usually this ends up heavier than a single adequately rated down bag. Getting from +30F to +10F will be a lot easier than getting from +30F to -20F.