I want to watercool my two HD7970 cards. Would a single 240 rad (e.g., xspc rs240) be enough?
Currently I'm running very gpu-intensive tasks and the cards are overheating (85+). The case is corsair 600T with a reasonable air flow.
You would be pushing it. Your first priority should be to keep the cards as cool as possible, especially during long tasks.
What you can do is figure out what the TDP output of a single card is, and check whether the radiator is capable of twice that.
Here's a review of your radiator: http://martinsliquidlab.i4memory.com/XSPC-RS240-Radiator-Review.html A bit of googling got me to 210W per card under load. Thus in the configuration in the review, you would need to run the fan at >3000 rpm constantly, only considering the gpus and having all else taken care of. So in my opinion, get a second radiator, or a bigger one.