I have had a pair of 780 Ti KPEs in SLI running for the last two months.
There was occassionally artifacting on one of the cards, however very recently one of the two would suddenly stop displaying, run its fans at 100% and the first three lights (voltages I believe) from left to right would go out. I removed the card that wasn't working however after a few minutes from boot, this also happened to the second card.
No heavy workloads or excessive temperatures, and a brand new 1200w Seasonic power supply was used.
Would baking them be the next best step?
Main Rig
Intel Xeon W3690, EVGA CLC280, 32GB G Skill Trident X 2133mhz/2400mhz, EVGA X58 Classified (E770), 780 Ti K|NGP|N Edition (2-way SLI), Asus ROG Thor, Noctis 450 ROG edition