80C is perfectly fine for a 680, especially a reference cooler that is factory overclocked. What seems counter intuitive is that the lower the resolution and easier the game is, the hotter the cards typically get. Higher FPS typically leads to higher temps.
Two monitors play no part in it, as two monitors only affects idle temps.
Reference designed cards will typically be ok with your card reaching nearly 85C before the fan starts to jump extremely high. If you want it cooler, you can use MSI afterburner and adjust the fan profile, but I'd leave it the way it is, or you will have some loud noises from the fan, and possibly wear the fan out sooner.