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First, set the SATA controller to RAID mode in BIOS, reboot and hit CTRL+I to enter the Intel RAID Configuration Utility. From there, setup a RAID-0 Array, save, and reboot with your Windows 8 installation media. It will recognized the RAID-0 array as one disk and you can install Windows there.
 
However, I recommend using RAID1 instead. You will get the same read speeds as RAID0, which makes booting and loading games fast!
 
An easy way to setup RAID1 is to install Windows to just one SSD (unplug all other drives). After installing drivers and updates, turn off the computer and connect the other SSD drive. After booting back into Windows, Use Disk Management to convert both SSDs to Dynamic, and then you can mirror each partition on the first SSD onto the second. The advantage of this method over using the Intel Utility is that you can import the drives into another computer easily, as the RAID1 configuration is stored on each member drive. With the Intel Utility, you may have to use expensive data recovery tools to recover data if one of the drives fails. 

Just for clarification RAID1 is not the same speeds as a RAID0.  RAID0 strips across the drives if the size of the data is larger than the striping size. Example if you have the stripe set to 64K and you have file smaller than 64K you will not see a speed increase.  However if you are throwing a game on there with HUGE maps and textures you will obviously see a huge performance increase.  The one thing to know is that RAID0 is faster than a RAID1 because the drives are striped, but there is no parity or fault tolerance.  If you lose a single drive in a RAID0 array you have lost all data on the drive.
 
@OP
You may need to install the drivers from a USB drive in the Windows 8.1 installer,  I had a heck of a time doing this I have to use the server platform drivers from the Intel website ( I think it was these, or something similar I do not recall: https://downloadcenter.intel.com/Detail_Desc.aspx?DwnldID=23257 ) which were newer & worked compared to the ones that EVGA had listed.  A RAID10 would be the best combination of speed / redundancy however since the x79 only has 2 native SATA3 ports you would have to use a RAID0/1 or use the native SATA2 ports since you said the drives are SATA2 then you could do a RAID5 which would have parity.  If you are interested in a good read look over at: http://en.wikipedia.org/wiki/RAID .  Any questions just let me know!