Originally Posted by gargoyle
Skip the zipper. It will add bulk and weight. And you'll need a beefy heavy duty suspension to support the combined quilt.
Rely on each individual quilts independent suspension. It will allow for more adjustable system, reducing chances of overcompressing either layer.
Stacking insulation items works well for varying temps, these items can be pads of varying thicknesses, down or synthetic underquilts.
I see the possibility of an exponential increase in fiddle-factor with two underquilts adjusted separately. If the zipper is used and the quilts are properly sized, the bottom quilt would be less
likely to compress the insulation in the top one. You're right that a strong suspension would be needed. Two separate suspension systems might be just as heavy, maybe heavier.
I suggest making the down (inner) UQ first and perfecting (
) its suspension before adding edge zippers and starting the outer primaloft quilt.
Virkkte, I think Gargoyle hit on the most important consideration when he said the weight would make suspending it difficult. Much depends on how your hammock is suspended and whether the UQ pulls on the sides of the hammock or is entirely supported by the ridgeline. Double underquilts use twice as much fabric, not 50% more.