This changes my backup planning for OSX.
Time Machine has gained full support for multiple listed target volumes; just add the additional backup drives in System Preferences. Time Machine will automatically rotate backup targets, using all drives alternately — or if only one is available, it will target that until the other drives come back. Restores default to using the most recent available backup set, as they should.
Today I have a SATA cradle (more on that here and a 1TB drive and a 1TB portable USB drive. Being able to use both at the same time, or just one when I am travelling makes good sense.