I think you're correct ... you won't get double but sure gonna be better than a single bag!
Comfort (and survival) will also depend on what is under you. A good insulating pad makes a world of difference. During two Antarctic tours I camped on ICE a few times, in support of Ornithologists doing Bird studies.
Oh Joy!  Once for a week.  Not a great time ... but it was work, no choice in the matter.
Temps were cold but not outrageous ... 10F (about -12C) We had Down Navy survival bags (best US military had in 1977) and Cots! I still froze my ass off.
I would not recommend Winter camping for any length of time. For me? emergency only. You can't ride in such temps, why would you camp there? Head South! (or North for Southern hemisphere!)
Rather than 2 bags, maybe use a Bag liner. They help a lot. Not sure how much they add to temp range but I'm sure it's noticeably better.
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