I'm not sure I understand what you intend to do:
-Having the Backup script use both disks?
Can't be directly done, the webUI only supports one backup folder. But after creating the backup definitions you can edit /etc/backup.conf and change /Backup to, e.g., /mnt/sdb2/Backup2. One backup can't be done across disks.
-Change the backup location to the new disk?
Change the current backup folder to the the disk, using the Change button under "Backup Destination Folder". Select, e.g. /mnt/sdb2, not /mnt/sdb2/Backup.
-Change and move the current backup folder to the new disk?
Use Setup->Folders to move (not copy) the current backup folder to the new disk, e.g., move /mnt/sda2/Backup to /mnt/sdb2 and reboot. The new Backup folder will be used.