If you turn off AA in BF3 and shadows off in WoW I bet you can run both games at once at 60FPS, maybe you will have to lower some settings slightly, medium at worst.
It's a goofy ridiculous idea anyway, there's hardly any reason to run two games at once. Battlefield 3 requires focus and attention, you can't farm goods and shoot at the same time. If you do have a good reason, I'd love to know.
On an older card I was able to run 4 team fortress 2 games at once fully maxed on one PC with only a without any significant performance drop, so I would take a guess and say they both those games can run just fine together on two screens(680 alone is way over kill for WoW); just use windowed mode.