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Why buy an Xeon when it's not even supported on that mobo...
TBH I think it's your CPU making the mobo throw a wobbly where SLI is concerned, your CPU will not be supported in any bios released by Asus for that mobo, if that has a direct impact on enabling SLI, who knows, stick a supported CPU in and see.
If both cards work perfectly fine on their own, I can't see how they are the problem.
I researched before buying, and there are a fair few who use Xeons on this motherboard without issues.
How would a Xeon CPU effect SLI not working ?
BIOS-basic input/output system, Exeon not supported in any Asus bios on OP's mobo, draw your own conclusions. First thing I would query if I was Asus tech support.
How would a Xeon CPU effect SLI not working ?
nVidia's system for checking if SLI is permitted on the hardware is a bit... "special"... i.e. possible scenario to "prevent" people strapping AMD chipsets to masquerade as intel ones to get SLI working they'd check if it was an i5/i7 CPU and if not shut out SLI support... as the xeon isn't officially supported it might not have been taken into consideration... this is all assumption tho.
Not supported in the bios on problem mobo. Why do you think mobo vendors release updated bios roms to support new CPU's, to provide increased compatibility.