New FamilyTalk add-a-lines have not been possible to order from Apple before so I don't expect it to work this year.
Just use the same process as last year, new single line and then merge the line into your existing FamilyTalk rate plan once you activate. You will need to call AT&T Customer Care or swing by a COR location.
Just make sure you know which line is the primary and that it does not get changed in the merge. Yes, you get to decide which line is the primary and yes it does affect your future upgrades. Think, highest cost means highest MRC, so put the line with the highest MRC as the primary line.
Dave