What this equation says, is that one of the layers will be compressed to 1/2 of its normal loft, and be less effective due to that.
This would obviously be the bottom layer.
This is also a bit non-scientific, probably overly conservative.
How much will a UL 14 oz 40F quilt compress a 30 degree bag? Not much at all. Possibly not any at all. Definitely not by 1/2.
However, for a heavier synthetic quilt on top of a UL down bag, it might do some.
The net result , is the heat transfer is by conduction , and it is linear. With 70F being "comfortable" with no insulation for people, the layers are additive in insulating ability. With the caution that dont expect perfection, you lose something.
Add it up, take away 10 or 20F if you like, or use the formula. Its just guesswork.
Just take something away, its not going to be perfect. The colder the condition, the more you may want to take away to be conservative.