ahh sure I thought it was the PCIe version, having that would solve your problem. anyway the manual wasn't clear but I found this thread http://www.overclockers.com/forums/showthread.php?t=647094
. Long story short one of the guys on that forum talked to ASUS and they said that when you use just slot 1 and slot 3 the PCIe slots will run x16,x0,x1 so you can't use the first and last PCIe for dual crossfire (only triple).
I have 4 possible solutions that may help
1) watercool your cards then they only take up one slot not two but that is expensive.
2) get a PCIe to PCI adaptor like this here
this may require some modding to your case PCI holder to make it look ok
3) get a PCIe x16 Extension cable like this one here
and then you'll need to find a way to mount it maybe on the random extra pci holder that some cases have.
4) get a new sound card or just not use it, I find that unless you are using the best speakers money can buy it doesn't make much of a difference with a sound card or not.
personally I would go with option 1 or 4 as they would look the neatest.