It doesn't work. MeDevil's kext doesn't support RAID yet. You can create a software RAID with the Disk Utility, but it won't be bootable. Though you can boot from an alternate disk, and set the root drive to your raid with bootflags.

http://osx86.wikidot.com/how-to-s#toc7

Use that How-To. When I did it, I didn't create the seperate boot partition, so I was using 2 whole drives. Instead I copied the boot files to a USB Pen Drive and used that to boot. Though, It was slow to boot, but it got the job done.