#316 split backup between two different targets

[HCH]

This has been raised in the discussion https://sourceforge.net/p/areca/discussion/587586/thread/ac2b3d6c/#5465.

The request is to have one backup of a target (e.g. a full backup) on one media/destination and another backup of the same target (e.g. a incremental backup ) on a different media/destination.

This would make ARECA more portable and a lot more flexible

a use case could be:

1. A full backup is made to a external HDD
2. on a trip/travel (to remote location without good internet connection), incremental backups to a usb would give you current backups without the need to carry a ext. HDD and the full backup would be more safe at home/in a data centre.
3. on return the backups could be merged.