Best approach to offsite backups with Time Machine and USB Disks
I have thought about that same problem for quite some time now. With online backups, large amounts of files are slow to upload and inconvenient to download; so I really like the two physical USB drives approach. Having both disks onsite while you clone the most recent disk is not as bad as long as you limit the time you use for this. Don't do it over a week, for instance. Get it cloned and offsite ASAP. 

Having said all of that, my answer to your question is to use something like a Drobo. I haven't done it yet, but Drobo seems like it fits the bill: The Mac will see it as one big Time Machine volume, but in reality you are mirroring multiple disks at the same time. If price is not an issue, and you don't mind handling SATA drives around (You can get an enclosure for the one that will be traveling offsite) it may be something to look into. 

Hope this helps.