I've been having issues getting SLI to work in both Windows 7,8, & 8.1. Both cards are seen by Windows and the proper configurations have been made in the bios. When trying to enable SLI in Nvidia control panel the options does not appear. I've tried several bench mark programs such as furmark but the second card remains idle with zero load and no gain in temp.
So far I've done the following:
1. Updated MOBO to the latest Bios
2. Uninstalled Nvidia drivers (Many times) Used driver cleaner, safe mode ect
3. Swapped Cards around tried running each card as a standalone (no issues)
4. Loaded Windows 7, 8, & 8.1 All behave the same.
5. Tried multiple SLI bridges
6. Tried multiple power supplies
 
My System:
Mobo: Asus Maximus Hero Vii
CPU: Intel 4790K
Memory: G.Skill Trident X Series 16GB 2400Mhz
PS: EVGA Supernova 1300W
 
Video: 2 EVGA 06G-P4-3787-KR G-SYNC Support GeForce GTX 780 6 GB DDR5 (In SLI)
I've tried uploading several jpg but the site will not allow it at this time.
 
I did notice that the memory type, device ID, & Bios version are different between the cards, I have a gut instinct his could leading to the issues.
Card 1:
Device ID: 10DE - 1004
Bios: 80.80.45.00.80 (P2083-0021)
Memory GDDR5 (Samsung)
Card 2:
Device ID: 10DE - 1007
Bios: 80.80.58.00.82 (P2083-0025)
Memory GDDR5 (Hynix)

Most likely not any minor card differences.  Different brands of cards will SLI.  Would have thought something to do with lane assignment but it appears that both cards are running at x8 according to the gpu-z.  Maybe check settings in bios for slot assignment.  When you tried the multiple SLI bridges I assume that you tried both sets of fingers.

Hello, teejayd
 
 We were happy to assist you with troubleshooting your issue also with taking in your GPU's for further testing. If you have any further questions or concerns feel free to contact our 24/7 Technical Support.

Wolvez, have you tried stress testing both cards individually? In teejayd's case, one card passed testing, the other card failed, so it may be the same in your situation that one card is fine, and the other is faulty too. Try looping Heaven or Valley benchmark for a few hours with only one card at a time in the system and see if they run fine.

@Wolvez 
 
As EVGA Support stated in the above comment one of my original cards was indeed defective however having said that, I did receive two new cards last week and am still experiencing the same behavior. I even went one step further and borrowed a friends system with working SLI (different cards) but was still unable to get them to work despite a plethora of other testing methods. 
A few things to note: Card #3 & #4 also had different Device ID's, Bios Version, & Memory manufacture.  From everything I've been told this shouldn't matter, but I'm still no 100% convinced.  
 
As it stands now I opted to do the advanced RMA so I will have 4 cards to test with this week.  Fingers crossed, I will updated after I've had a chance to test. 

teejayd
@Wolvez 
 
As EVGA Support stated in the above comment one of my original cards was indeed defective however having said that, I did receive two new cards last week and am still experiencing the same behavior. I even went one step further and borrowed a friends system with working SLI (different cards) but was still unable to get them to work despite a plethora of other testing methods. 
A few things to note: Card #3 & #4 also had different Device ID's, Bios Version, & Memory manufacture.  From everything I've been told this shouldn't matter, but I'm still no 100% convinced.  
 
As it stands now I opted to do the advanced RMA so I will have 4 cards to test with this week.  Fingers crossed, I will updated after I've had a chance to test. 

This is not meant as a jab, but I am curious as to why you think it still is the cards when you put a known working SLI setup into your computer and it does not work?
I see there is a question mark by the PCIe section in gpuz.  Do you have the setting on auto in the bios.  Try setting both to to gen3 if available.  If not set to gen2.

Why not NVFlash_Windows_5.165 (extract the .zip to desktop) at your own risk so that both cards have the exact same VBIOS just to find out if that's the problem?..Use GPU-Z to save the VBIOS .rom file (click the little BIOS chip icon next to BIOS version) in the nvflash_windows_5.165 folder (on your desktop) from the second card and then shut down and take the second card out..Start up and open NVFlash then drag-n-drop the VBIOS .rom file that you saved from the second card into it and seeing as how they're both the same type of cards, NVFlash should auto-flash your first card to the VBIOS you saved from the second card, then reboot..Then shut down and put your second card back in and try it..Verify that both cards have the exact same VBIOS with GPU-Z..If that fixes it, you can leave it be if you want to, or not and RMA, I understand that you should want your cards working hassle-free as is..But remember if you RMA it you'll need to NVFlash back to the original VBIOS in the same manner..This has worked for people before.
 
If you want to, you can leave both cards in during an NVFlash and choose which card (0 or 1) you want to flash when the prompt appears, but you might feel more comfortable doing it with only one card installed (suggested)..You can also save a copy of the VBIOS using NVFlash commands, but using GPU-Z is more simple..You will be NVFlashing within the Windows OS environment..If it doesn't auto-flash or if you have any problem(s), you can do it manually and rather than me typing out the whole proceedure, Sajin has summed it all up here.
 
Just a thought.

I'm having the exact same issue.. 
 

Hey Teejayd, I hope it works out so we can resolve the issue. :) Thanks for doing this!

Having the same exact issue as well with a (new) 6GB 780 I just got yesterday to SLI with the one I got back in March. Did everything in the book to try and get the SLI option to work and no dice. My GPU-Z looks exactly like Falterx screenshots as well.

After testing them separately, the one that's running on the Samsung memory seems to be performing consistently worse (1k score off in Furmark compared to the other one, 3173 vs 4292). Considering the fact that so many people seem to be having the same issue, it's hard to believe that the device ID or different memory types are not the issue as EVGA tech support suggests, according to them the performance difference is an indicator of a faulty product which would cause failure to SLI, but I guess I'll find out next tuesday when I get my new card (or when Teejayd posts his update, hopefully today) 

Update:  Spoke with Tech Support again this morning and they are telling me that this is an NVIDIA driver issue, not a hardware issue.  They have not had any luck with the RMA process and users getting this issue resolved.  So the final verdict was sit tight and wait for a driver update.  Riiiiiiight.  He did tell me that attempting to flash the bios would not void my warranty but declined to share if any of these issues had been resolved by flashing the bios.  Furthermore, he told me that one guy had success by rolling back to the 335 drivers from march.  So this is where it gets interesting.
 
-Even though the newest update of GPU-Z shows both cards as Geforce GTX 780, NVFlash does not.  It shows the original card I got on Step Up back in may as a GK1XX (thats not censored, it actually shows that.)
-Attempting to roll back the drivers to 335 installs my newer card perfectly, but the GN1XX is not recognized as a GTX 780 and only loads the Standard VGA Drivers - even when I attempt a manual install of the driver it fails, telling me that this version of the driver software is not supported by this version of Windows.  Wow.
 
So here is the question for anyone that has attempted a BIOS flash. Will it change the way Windows sees the card?  I believe this to be the underlying issue as to why these won't sli.  As far as Windows and the NVIDIA drivers are concerned, these are not identical chips and not capable of being SLId.  
 
Just that the bios flash is risky.  If this chipset on the original card is janky, might brick the card on flash.  At least now I have the option to try to sell as is and step-up the newer card.
 
Sooooo, anyone attempted a bios flash?

Cant find any option for drag and drop. The only nvflash I can find has to be done through admin level command prompt.

On Friday I received RMA cards 5 and 6 from EVGA.  These cards also had mismatched memory manufacturer, Bios, and device ID's. Despite a clean sweep and reinstall of the drivers I still experienced the same behavior as the previous 4 cards.
 
With that said, as mentioned above I oped to do the advanced RMA so I had in my possession 4 cards. Within this group I was able to find two cards with the same memory manufacturer, Bios, and device ID's.  After installing the two Samsung memory cards, I was able to configure SLI from the Nvidia Control Panel.
So as suspected from the beginning, there seems to be an underlying issues between EVGA's cards using Samsung and Hynix memory. 
 
EVGA - Please look into this issue further, it seems to be impacting multiple people.
 
 
 
 

bphillips9900
Must have been misinterpreting something in your post...I was looking for a GUI or something. Nonetheless, here are the results of the manual flash, which required a lot of scary overrides.

-Sucessfully flashed the newer bios onto the questionable older card
-The update did not change the device id or the way Windows recognized the card
-Could not boot into Windows with the newest GTX drivers, just black screen over and over even though the newly flashed card was in the second slot. Even pulled the sli bridge and still couldnt boot
-Booted safe mode and change flashed card to standard vga adapter
-Got a good boot and flashed questionable card back to original bios
-Got a good boot with GTX drivers on both cards

Sooo, no brick(yay), but no resolution to the issue either. Right back where I started. Activated Step Up on the new card and the old one is listed on a popular auction site. I'm done...adios 780 6gb. What a lame experience. Thanks for $1200 worth of garbage, EVGA.

Good morning bphillips9900,
I am sorry to hear you are having troubles. I can confirm that our product team is looking into this issue and should have a follow up early next week. We are here to support you with warranty if you would like to start the RMA process. Please feel free to call us 24/7 at 888-881-3842.