I believe that most of the heat disapation comes from the air flowing through the vents inside the rotor. Also, the second caliper might use a pad about 1" in diameter, round pads are common in this application, so it's not losing very much surface area.
I don't think the material of the rotor matters too much, but I could be wrong.
this is the kind of caliper I had in mind
http://www.wilwood.com/products/calipers/msc/index.asp