I wouldn't apply a 2nd layer of insulation. The side of the foil (that you can't see) will 'reflect' heat back towards the duct to improve effeciency. The 2nd layer won't do much since the foil from the FIRST layer will simply 'trap' the heat that 'excapes' from it.
If you want to add more insulation, there are insulation suppliers in the YELLOW PAGES. Contact them to see if they make a 1" insulation, or something along those lines. If it make any matter, I have 1/2" insulation on 2 ducts that run through an UNHEATED crawlspace over a distance of 15' to a register (on the far side of the room.) Although I'd like a thicker insulation on those ducts, the 1/2" insulation is pretty good. You get a very short 'blast' of cold air but it's quickly replaced w/warm air. So, in your case, the 1/2" should be just fine. BUT, if you want more, I'd check into something that requires only ONE layer of installation; not 2 or more.