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?

Seems very Odd to lose two GPU @ exactly the same time

You changed your PSU cables when you switched PSU ?
 
Could it be a MB issue ?
 
Does your MB have Aux PCIe power port ?  Are you using it ?
 
1) Clear CMOS (after you snap some pics of your settings)


2) Test with no OC on CPU / MB / RAM (do not use XMP)
 
3) Test in different PCIe slots (you get power from PCIe also up to 75 W)