Depends what "better" means to you. I'd guess power/performance? In that case, the 780s would outperform the 880. But make sure you have a compatible computer. Like a motherboard that supports SLI, and a power supply that can give out enough watts (Probably at least an 750W, 80+ Ceritified, and 42A on the 12v rails.)
Adding another 780 probably will increase the FPS by like 33% or so, but already, the 780 can handle any game with 60+ FPS; So the change with another 780 isn't really going to make the game a lot smoother I guess.