Raid 5 does this, it uses 2 drives for stripping (Raid 0) and a 3rd drive for parrity data so if you loose one of the drives you can recover. But, it's not a true backup for the data really, more of a disaster recovery, which maybe what you are looking for.
People often use "backup" and "disaster/data recovery" as the same thing. Backup is when you have a file and you need to find an older version, or you deleted a single file or a directory. Disaster recovery is what RAID is used for, when a hard drive craps out on you and you need to restore it fully.http://en.wikipedia.org/wiki/RAID
So if you want a "backup drive" you need to setup RAID 0 on the 2 drives, and run a backup software to save to the 3rd drive. If you are looking for "disaster recovery" in case one of your disks fail, use RAID 5.