I dont think you can do this but why don't you/can't you set the first X number of gb on the ssd set to what you need to boot up, while the rest on raid (might have to have certain part of OS for it to work right)
partition 1, drive 1(let's say 6gb, or whatever it ends up being): what is needed to boot up the computer (whatever the minimum technically you need to boot up)
partition 2, drive 1+2(example: 250gb) in raid:all your stuff that you want in superspeed mode
partition 1, drive 2(let's say 6 gb on drive 2 that isn't in raid): stuff like office, some music, whatever you can put in it that doesn't need speed boost
