I've thought before that using a spare hammock to snug up a blanket, old sleeping bag or a quilt (down or otherwise) would be a fine idea. Naturally after looking this up on these forums others had also considered this. I don't have a formal UQ so this weekend when I decided to sleep out on the porch during some cooler late summer nights I thought I would give it a go.
My sleeping hammock is a War Bonnet 1.7 DL Traveler. My bottom hammock is an ENO DoubleNest that I took from my supply. The insulation layer was an old quilt that the wife had laying around. Just some crappy one that we use to throw down in the grass when we are at ballgames or whatnot.
To keep the quilt in place lengthwise I loosely tied one corner to the head of the hammock and one corner to the foot. I then lay in my Traveler to gauge where the DoubleNest needed to be. I hooked the DoubleNest up right to the ends of the hammock the same as my insulating layer. A few ups and downs in my Traveler and I figured I had it as best I could.
In the evening I got in my hammock and made sure my insulating layer and the DoubleNest were not in good position so I could see how things felt. Being at the beginning of the evening it was pleasantly cool. After about 5 minutes I reached underneath and adjusted everything into position. The on thing I noticed right away was that thought I could pull the planted up over top of my hammock I knew that as soon as I shifted they would fall off. I went ahead and nudged them until they did just that. By that time (15 seconds?) I could feel the added warmth on my back and rump area so at least it was a partial success.
Later in the evening ..... say 1 am or so.... I had to "get up" for duties and again lay down and made sure the insulating layer and DoubleNest were not in good position. Chilly, but certainly bearable. I put everything in place and I warmed up right away in my back and rump area. After a few minutes I pulled the extra quilt up and over my traveler and that enabled the sides to warm up a bit and also help the top.
I think for early Fall this setup would do "as is" but not much beyond that. Modifications would be needed to extend using it further into the season. I'd have to tie on some string every 18" or so to the quilt and hook it to my ridgeline. That would snug everything up along the whole length and keep the quilt in contact with the sides. It would be a major pain to manage this especially when you are still a bit sleepy in the middle of the night trying to get in and out. Otherwise a few carabiners on the ridgeline would make it simple to get in the hammock and reach over and pull up the attachments and hook them up.
I certainly think a formal UQ is superior but if you are looking to "get by" for a bit with what you have you can make it work. The next time I sleep outside I am going to hook the sides to the ridgeline just to see how it works out. If it fails and I get cold I can just slink back into the house and crawl into bed.