I've got two quilts for colder weather, one down (Golite Ultra 20)and one a DIY climashield XP 5osy quilt. Either would be fine if the weather were above 20 degrees. That's the kind of weather I'm used to here in Missouri in the winter. But recently the overnight temps have been much lower, like teens and even single digits. So I'm thinking of using both quilts with one as as "overbag".

Then I began wondering which should be on top? One the one hand I would think the down quilt should be on top to keep the down fully lofted. On the other hand the climashield quilt would be less affected by any condensation that might get on it. (Although I'll be tarping so I'm not really expecting any condensation).

Any arguments one way or the other?

in colder weather you can get condensation in the middle of the insulation, as that is sometimes where the dewpoint is. So having the synthetic abg on the outside makes sense, since it's the outside bag that may have condensation within it's insulation.

I believe that's what Phat does in the wilds of Canada.

I generally use a vapor barrier in below-freezing weather to keep moisture from condensing or freezing inside my sleeping bag's insulation. It works fine for me, but I know it's not for everyone. We all are quite individual in how we react to these things.