If I use two identical 20-degree-rated bags, one inside the other, what would likely be the effective temperature rating? 10-degree? 0-degree? negative-10-degree?
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Warning This is pure conjecture! Test this information with your gear in a safe place before using!
That being said, the basic warmth of a sleeping bag is determined by the amount of loft the fill provides. Quality of fill has a huge impact on how much loft is necessary to provide a given warmth, along with construction (i.e. baffles vs sewn through, hood quality, down distribution).
Putting all that aside, a good 20F down bag will have 4-5" of loft. So theoretically one inside the other would have 8-10". Again, theoretically that would give you ~ -10F (or perhaps even lower!).
This is assuming you don't have any down compression happening when you stuff one inside the other (i.e. you choose two bags with girths such that they don't compress each other). Since that's not likely possible, I would imagine real world performance would be quite a bit lower (maybe -5F?), and I would certainly want to test it with backups available before striking out into the wild!