I have used double batting a couple of times and really like the look. Green Fairy does it a lot.
I also get warm very easily but don't have a problem with poly bat. In fact my summer quilt is poly batting but just a single layer. I love the look of two layers of batting but I would think two layers of cotton would be heavy. I have mixed bamboo and poly and 80/20 and poly. Greenfairy has done two layers of 80/20.
Another option you may want to consider is trapunto. Very labor intensive with the cutting away of the extra batting but a beautiful look as well.