Make Time Machine use two drives from Terminal
I don't think using terminal to configure Time Machine to back up to 2 different disks is necessary.  If you have Time Machine setup currently to back up to one external disk, you can go back to the Time Machine preferences in System Preferences and select a different drive to back up to.  After doing this you will be presented with the option of using both drives for Time Machine backups or change the existing drive. If you select the option of using two separate drives, the Time Machine backups will alternate destination drives for each backup.  At least these are the options I am presented with, while using two separate 3 tb Time Capsules.