A straight doubling of the 680's 195 W TDP is 390 W, which is just 15 W more than the 375 W limit of 2x 8-pin. Given that dual cards seem to have lower than 2x the TDP of the corresponding single card, wouldn't it be possible to use two 680s without any core/memory spec reductions?

Although I don't quite know how the 650 W PSU minimum fits in here… it's between the 580's 600 W and 590's 700 W, so maybe the 690 will have closer to 300 W TDP than 375 W?