Command Line method to backup using two Time Machine disks

Solution 1:

Note: The restore component of backing up like this has not yet been tested.

Get the ID of the attached drive

defaults read /Library/Preferences/com.apple.TimeMachine BackupAlias                       

Eject, plug in 2nd drive, make it the Time Machine drive with the GUI, repeat.

Write (rather than read) the ID:

defaults write /Library/Preferences/com.apple.TimeMachine BackupAlias '<00000000 014e0002 ....>'                                                   

source & thanks to Christian Stocker. Key comments from that post:

1. http://developer.apple.com/DOCUMENTATION/Darwin/Reference/ManPages/man1/defaults.1.html "WARNING: The defaults command will be changed in an upcoming major release to only operate on preferences domains. General plist manipulation utilities will be folded into a different command-line program."

2. Also the property DestinationVolumeUUID should be set.

Solution 2:

If your need is to have redundancy on Time Machine backups, an alternative is to maintain a clone of your Time Machine Volume. I use SuperDuper! for that purpose.