Yes, but unless you put the 2 drives on separate FW ports you won't really be getting much in the way of a speed boost because putting two drives on the same port will be bandwidth limited by the port. You do the array in Disk Utility. Not that the risk of loosing your data increases by writing to the two disks with no redundancy...you loose one drive, you've lost your data and drive recovery services are very expensive for recovering data from any array set up except mirrored or spanned.
Isn't RAID 1 mirroring (not striping, RAID 0)? Then it's about redundancy, which you would get, and not speed, which is what Jeff is talking about.