I'm running into a weird problem. Tried to install a R9 290x together with a GTX 980, the former was installed on 3rd PCI-e slot and the latter on the 1st slot. For some reason the card installed on the 3rd slot does not work, Windows device manager shows an error "The device cannot be started". So I tried to swap the cards and the R9 290x got recognized by Windows when installed on 1st PCI-e slot and the GTX 980 on the 3rd slot did not. The error message on device manager is the same: "The device cannot be started".
 
Both PCI-e slots are enabled at BIOS, 1st is PCI-e 3.0 x16 and 3rd is 3.0 x8. By the way, I remember I was able to setup both slots at x16 but with latest BIOS for X79 SLI I cannot see the option to change the speed anymore. 
 
Any suggestions?
 
Thanks.
 
PS: before someone asks why do I need to have both an AMD and NVIDIA video cards installed at same time, here's why: I need to test games running on both architectures and having them plugged is much more convenient than swapping cards back and forth. Just have to change which card is attached to the main monitor and play the game.

Maybe I'm wrong but can this be done? I've never heard of anybody running two distinctly different GPU's on the same motherboard. Sometimes running SLI or Crossfire can be difficult enough to get running just right let alone using two completely different graphics cards. My suggestion is don't do it, but again, maybe I'm wrong.

Removed

AMD and Nvidia are intentionally designed to be incompatible with each other and cant be run in crossfire or sli together because of this reason. Cards can only be run as a single card only when pairing amd with NVidia. This explains why your having conflicts, and this cant be fixed. You may have performance conflicts down the road even though it may seem to be working properly. No catalyst driver ever has been compatible with NVidia and vise versa. These cards will also run your desktop at fairly high resolutions without the use of any driver installed and can play games as well with no driver installed at all. One of your cards is most likely not using a driver. What your trying to accomplish is causing more harm than good.   

bcavnaugh
Why would run a AMD and a NVIDIA Graphics Card in the Same Computer?
 

mautari
PS: before someone asks why do I need to have both an AMD and NVIDIA video cards installed at same time, here's why: I need to test games running on both architectures and having them plugged is much more convenient than swapping cards back and forth. Just have to change which card is attached to the main monitor and play the game.

 
Testing Games?
How do you tell a Game Which Graphics to use?

The game runs using the graphics card attached to the main monitor. No need to tell the game which card to use :)
 

VIPERIXIX
AMD and Nvidia are intentionally designed to be incompatible with each other and cant be run in crossfire or sli together because of this reason. Cards can only be run as a single card only when pairing amd with NVidia. This explains why your having conflicts, and this cant be fixed. You may have performance conflicts down the road even though it may seem to be working properly. No catalyst driver ever has been compatible with NVidia and vise versa. These cards will also run your desktop at fairly high resolutions without the use of any driver installed and can play games as well with no driver installed at all. One of your cards is most likely not using a driver. What your trying to accomplish is causing more harm than good.   

I'm not trying to use both at same time let alone running them in SLI or Crossfire, which would be laughable 
 
No harm done, I'm using them on a daily-basis and works flawlessly.
 
Thanks for sharing your thoughts, though.
 
 

 
Why!
 
EDIT*
Looking back on this image, this is how I sometimes feel when I post on EVGA now that I have AMD cards myself...