I will recommend you to roll back to windows 7 just to make sure is not related to software, if the issue persists you can start by replacing the SLI o Crossfire bridge and if that doesn't fix it I would recommend you to replace the video cards. 

have you tried updating the video card bios? one at a time since they seem to work

but  I would backdate back to windows 7 to verify all is good as stated.. not  sure which  video cards you have but as an example and  if   you can post your video card model

 I see two AMD Radeon HD7970M Wimbledon XTs, one is designated for Windows 8 and has a different Part #  so this might be  your issue aswell .. but since  I don't know what cards you have this is just a guess

yes they are both 7970m , they both worked in windows 7 ultimate and 8.1 until I updated the bios on the mainboard, I have the same issue in windows 7 pro, ultimate , 8.0 and 8.1 pro.  

I have swapped both cards from the primary to the secondary and back and the issue is the same , the cards stay cool and the fans run in diagnostics on the motherboard and pass dells online tests also.

But when I load any Os they both take turns getting hot , the fans don't spin up except on the CPU , I can get one card on the primary to work alone and the drivers to load but when I add the second card into the second slot the drivers will not load and then I get a crash , either black screen or a color similar to whatever wallpaper I have loaded on the desktop both Gpu's work alone and pass all the onboard and dell tests.

Now , I have blindflashed the board back to a08 from version 12.   I have more diagnostic showing in mainboard but same results no issue found until I load an OS.   I cannot for the life of me figure out whats wrong.

I do not have any experience doing the bios on the Gpu's  and any help would be appreciated.  I cant afford to gamble at replacing the mainboard and then find out it was a Gpu or vise versa.

What do you mean the cross over bridge ?   

yes they are both 7970m , they both worked in windows 7 ultimate and 8.1 until I updated the bios on the mainboard, I have the same issue in windows 7 pro, ultimate , 8.0 and 8.1 pro.  

I have swapped both cards from the primary to the secondary and back and the issue is the same , the cards stay cool and the fans run in diagnostics on the motherboard and pass dells online tests also.

But when I load any Os they both take turns getting hot , the fans don't spin up except on the CPU , I can get one card on the primary to work alone and the drivers to load but when I add the second card into the second slot the drivers will not load and then I get a crash , either black screen or a color similar to whatever wallpaper I have loaded on the desktop both Gpu's work alone and pass all the onboard and dell tests.

Now , I have blindflashed the board back to a08 from version 12.   I have more diagnostic showing in mainboard but same results no issue found until I load an OS.   I cannot for the life of me figure out whats wrong.

I do not have any experience doing the bios on the Gpu's  and any help would be appreciated.  I cant afford to gamble at replacing the mainboard and then find out it was a Gpu or vise versa.

ok heres an update , I updated the bios up to 10 there are 2 left 11 and 12 . I installed windows 7 ultimate again , it loads the drivers this time but totally black screen when windows reboots and loads.  The video cards both start overheating and the system freezes no hard disk activity.   ive tried everything at this point I either have a bad Gpu or mainboard.  But both gpu work alone and the system works great with one card also.  I don't know what to replace !

I have no idea how to do the vbios because I cant find anything on updating the vbios.

okay sorry for delay but some reason I cant find the link on the forums and one I got saved says page cant  be found :(   maybe someone from dell  have the link for the vbios download  and the  info sorry  :(

also  if  you download gpu-z you can find out your  current bios revision 

Normally to do a Video card Bios update you need to download an ISO file burned into a cd and then boot from that cd; however we don’t have any VBios update in the Dell support website so you might need to search it in the AMD website. 

if he can get gpu-z and get bios version i can find out if its current, if not il search for the recent and compare to my 7970M before i ship them back