I've only stacked top quilts, a 20 and 40. It was a ridiculous puffball of down. Felt like I could have gone to -20°. Maybe more since my 0° alone was plenty down to -16°.
Good point about compressing the UQ. It would definitely be a good idea to test the fit before actually doing this.
I tried it years ago and it looked like a mess, but I think it would have worked just fine (don't over tighten ends like this)
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