In an ideal world they should be in different locations to each other, to reduce the chance of loss.They just sit in a cupboard until needed.
So can stuff in your house, belt and braces use both, unless you move your backup to another location away from your property then your backup isn't safe
Fire, lightening strike, flood, theft are all risks as are mechanical failure of your extra drive
Not for business just personal stuff. I've just finished backing up all our photos, letters etc on to two different hard drives. They just sit in a cupboard until needed. Usually do it every year when I'm bored at Christmas!
If it's a mechanical / circuit board issue with the drive, could you not swap over the disk to another (same make etc) drive?