If you go to the Time Machine System Preference and click on Select Disk, if you already have a disk set as your backup and you select a second disk, you are presented with a dialog asking if you want to delete the current disk or use both disks for Time Machine. You are told that if you us both disks, Time Machine will take turns backing up to both disks. Nice touch to allow backups at work and home to happen at the same time!
[kirkmc adds: This is one of the features that Apple mentioned about Time Machine in Mountain Lion. The backups rotate, and you can either use disks at home and at work, or even two disks in the same place if you are paranoid about backups (as I am).]
Mac OS X Hints