tucker147
It should work fine with one 8 pin to 2x 6+2 pin cable per gpu.

bob16314
Welcome to the forums.. is a very good article about Power Supplies, Voltage Rails, PCI Express Auxiliary Graphics Power Connectors and more.
 
The good news is that a has a single +12V rail with 70.8A..The answer to your question is yes, one 8-pin that's split into two 6+2-pin connectors will work for each card no need for seperate ones..You're good to go.
 
EVGA GTX 780 Ti SC's are 250W cards by the way (see ).

bob16314
You're welcome.

bob16314
You should make sure you have the latest BIOS for your mobo..Try clearing CMOS by either the mobo switch/jumper or by unplugging the PSU and removing the CMOS battery for about 10 minutes, make any custom settings changes again upon startup..That's two of the most common simple fixes..You should make a signature (go to User Control Panel at the top of this page) listing your hardware and Windows 8.1 

bob16314
Try installing the driver again..The cards need to be the same in some respects, see (could be one of your cards is slightly different and that's why it's not showing up in the NVCPL)..Make sure the card is seated all the way in the slot..Make sure the 6-pin and 8-pin are snapped all the way in the card and the 8-pin in the PSU..Try the card all by itself in the top slot to see if it's DOA or not..Other than that, hmmmm.

bob16314
Okay, now try just one card in the second slot, leave the first slot empty..I don't think it matters on your mobo which slot a single card is in (it don't on my mobo anyway, and yours is alot newer), if a single card won't work by itself in the second slot then  because the slot is probably dead.

bob16314
Hmmm, interesting, and thx for the update about how you got it working.
 
But, lemme get this straight now (not trying to be air-headed, but have my moments sometimes, lol)..You have ONE 8-pin splitter cable (i.e. 8-pin connector on PSU end with two 6+2-pin connectors on the VGA end) as shown in the pic in your first post going to the top card, leaving only a +2-pin connector hanging from the card's 6-pin socket..Cool.
 
On the bottom card you have TWO 8-pin splitter cables going to it and using one 6+2-pin connector from one cable plugged into the card's 8-pin socket (leaving the extra 6+2-pin from that cable hanging)? Then you have the another 8-pin splitter cable plugged into the card's 6-pin socket (leaving the extra +2-pin and the 6+2-pin from that cable hanging)?
 
Is all that correct?
 
If that's correct, not only would that be cluttered/unsightly/unsat to some, but something doesn't seem quite right that way..I know that "if it works, it works", but unless I'm missing something, only one 8-pin splitter cable should be needed for each card..And since that PSU comes with three 8-pin splitter cables (for a total of six 6+2-pin connectors), the way you have it connected with the supplied cables would rule out the possibility of a tri-SLI setup unless you bought extra cables, and that would also be not right/unsat with that quality of a single 12V rail PSU.  
 
Like I said, something don't seem kosher, just trying to understand what it is..Have you tried a different cable or 6+2 PCI-E output from the PSU using only one cable to the bottom card? (thinking faulty PSU output or one bad cable out of the three supplied cables).