I'm guessing two 780s in SLI will outperform the 880.But the 880 is probably going to be stronger than the 780.
The rule about GPUs is,you get the better card in favor of 2 weaker cards in SLI,and then you SLI the better card in the future.
SLI does suffer from stuttering,draws more power,generates more heat & noise and not many games are optimized for SLI.
Since you already have a 780,it would be cheaper to get another and SLI(if your current PSU and mobo support it ofc).
If you've got good cooling and a decent psu(and can put up with a bit of stuttering),go for SLI 780s.
But,do you really need that much power ?