This will work but you temporarily need a third drive.
Add two identical drives to be used as your new RAID volume that you want to boot from. Leave the original boot volume intact for now. Set up the 2 added volumes as a single mirrored raid volume and format them. I would suggest a low level format; this is a very slow process prior to the actual formatting that writes zero's to the entire drive.
(You can typically get by without doing this on a Mirrored volume but on a raid 5 or 10 etc it's pretty much a must do or you'll have disk errors down the road.)
Then "clone" (freeware clone software is available for both Macs & PC's) the boot drive to that new mirrored volume. In doing it this way the boot sector etc are copied to the new volume.
Now remove the original boot volume and you'll be able to boot from the new mirrored volume. This works great on both Macs and PC's.
As a further note 2 drive and 4 drive Raid enclosures are available to day for 200 bucks or less that are decent but not enterprise.