You don't have to worry about 2 drives crashing at the same time unless you get a massive power surge or a flood or tornado or something. The easiest and safest (IMO) way to store backups is to simply backup to an external hard drive, then store that hard drive unpowered in a safe location.
I know that for Clonezilla, and I believe for PartImage as well, the backups need to all be contained in 1 media (i.e. on 1 harddrive, 1 dvd, 1 cd, etc.)
You can always split the files using a command like
and burn them to multiple dvds, then if you ever need to restore from your backups, copy the split parts back onto an external harddrive and recombine
split -d -b 4000m /path/to/backup.tar.gz /name/of/backup.tar.gz
cat *tar.gz* | tar -xvpzf - -C /