"(2-tier backup better than 1-tier, right?)" - there's the catch: if it's automated, and you accidentally delete your files on one of the macbooks, the delete w

"(2-tier backup better than 1-tier, right?)" - there's the catch: if it's automated, and you accidentally delete your files on one of the macbooks, the delete will propagate to the other macbook, as well as to the hdd. Dropbox offers synchronization, which is not backup, IMNSHO (it also offers a form of limited backup, keeping the 30-day rolling file history in the cloud, but that's independent of the number of synced devices). I'd go for rdiff-backup - based on rsync, last version of the backups is accessible without special tools, previous versions can be retrieved using rdiff-backup; plus a daily cronjob to sync it with your dropbox folder.