Currently Being ModeratedJul 18, 2013 2:45 AM (in response to Ostia)
See Pondini's wonderful Time Machine Tutorial for answers to your question and other TM advice:
Currently Being ModeratedJul 18, 2013 4:00 AM (in response to Ostia)
I don't believe that is how it works. You can add multiple disks, but they are just redundant backups, not an extension of the previous backup. But, if there is a way, Pondini would likely know as nbar has already pointed out.
Currently Being ModeratedJul 18, 2013 9:52 AM (in response to Ostia)
Instead, when I add the new 6TB disk it wants to back up the old 6TB onto the new one
If you mean that TM is going to copy the backup data from the old drive to the new one, no, that doesn't happen. If you have non-backup data on the old drive, and you've removed it from the list of excluded volumes, then that non-backup data will be backed up to the new drive, as it should be.