If two drives are tied together as a JBOD, one can fail and you can still recover the data from the good drive.
However by using two drives you are increasing the likelihood of failure. With one drive, you have (say) a 1 in 50,000 chance of having a hard drive failure on any given day. Now since you use two drives, you have a 2 in 50,000 chance of having one fail. You're better off with the odds of using only one drive, since it has less chance to fail, and making sure it is not your only back-up.
Backups are best done in pairs and then one copy removed from site.