I have seen a post or two from people that have combined quilts to boost warmth. I am thinking of doing something similar with a 40 degree and a 20 degree quilt that combined inside a Bivy would give me zero degree coverage. Interested to hear from a few more people that have done something similar and how effective it was.
The alternative would be to go with a zero degree quilt but for me I think I would get a lot of use in shoulder seasons from a 20 degree quilt by itself and would have relatively few occasions to need zero degree coverage. Therefore the idea of a combination approach would seem to be smart unless there is something I am not thinking about.