You do get an option to transfer additional characters on the same screen as the transfer request confirmation.
Originally Posted by Morghane
Q: Will our ACCOUNT names be the same?
A: Your account name will be the same name as your 12FootTall.com portal account name.
...leads me to believe that if all the characters you transfer using a single 12FT account will be on that account, which you will then use to sign into Lunia. Makes me wonder what will happen if you attempt to transfer more than 4 characters.