I have the MSI Twin Frozr II OC and my typical temps are in the low 60's when I've been playing for a while. In terms of maxing out temps, video cards tend to handle more heat - while 80 is high, I believe the 6950 can safely reach as high as 85-90.
You probably know this, but you might try manually setting the fan speeds to improve airflow. Also, do you have a case with good airflow? You need a side fan to bring in cool air over those cards, especially with a CFX setup.
I'd also blow/vacuum dust out of the top card, just to be safe. CFX setups will generally produce more heat, but the difference in the 2 cards seems a little odd.