I'm not saying they run twice as hot, I'm saying they use twice as much power and therefore release twice as much heat. That's not the same as the temperature doubling.
Firstly it's temperature above ambient that needs to be considered, not absolute temperature. Secondly the approximation that temperature varies linearly with rate of heat transfer isn't great. Finally this is approximate, as only the cores are doubled, the rest of the circuitry is the same. I'm assuming it's the cores that use the most power, which seems a fair guess.
The drop to 32nm means the increase in power consumption will be less than this implies though, as the above assumes constant efficiency.