Hi and welcome to the forum. Good to hear from another Georgia hanger.
I have'nt heard of anyone using two tarps like you talk about. The weight penalty would not be worth it IMO. Plus other negative factors I'm sure.
If using a Hennessy, most people here upgrade to a bigger tarp. The MacCat tarps are popular. The Sportsmans Guide tarp is pretty big with a 12' 8" ridgleine and is hard to beat for the money:
www.sportsmansguide.com/cb/cb.asp?a=97247
No need for an underquilt in cold weather. They are nice and I own a couple, but closed cell pads work well in hammocks if you are not prone to sweating issues with pads.The Walmart blue 3/8" pad is very popular. I've used just pads down to 10 degrees and had a blast.