I’m guessing you want to backup FROM multiple drives and not backup TO multiple drives. Is that right?
If so, your screenshot is from the backup “Destination” (2) page and not the backup “Source” (3) page of the new backup configuration. You specify the things you want to backup on the next screen (3). As you can imagine, the “Destination” page is where you define where your backups should be stored/sent.
Once you figure it all out, Duplicati is awesome!