"(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.

rsync -az --delete ~/dropbox/ /volumes/myexternalhdd/

Treat Dropbox like RAID rather than a backup (i.e. RAID is not a backup). I, personally, have a set of rotating hard drives that snapshot data over a month's time. I also buy cheap HDDs that are frozen and forever hold data from a specific date, back a few years. Duplicates are held in a safety deposit box.

Example of cheapo hard drives (I use them in pairs): http://www.newegg.com/Product/Product.aspx?Item=N82E16822136...