Striped/redundant/fault-tolerant backups

This theoretical feature would allow a backup set to be spread across multiple targets. Several advantages:

* This could be used in situations where space on particular targets is not enough for a single backup set; a serial, chained set could be spread across several targets. (All targets would need to be available to operate on the backup set, though. But maybe the user is okay with that and could use such an option.)

* Going in the other extreme, a full backup set could be created on multiple targets at the same time for complete redundancy. (This is something *I* could use immediately, since today I do this manually.)

* More complex: If a backup set were divided across multiple targets with an overlap strategy, this could give the benefits of redundancy while reducing the space needed on each target. For example, a backup to 3 targets could be divided up so that only 2/3 the space of a full backup is used on each target, and a restore could still be done even if one of the three targets could not be reached. (Maybe give the user the option of deciding how much redundancy is needed-- e.g. 2 out of 4, 3 out of 4, 3 out of 5...)