Normally, TurboTax Online is billed per return, so you are charged $34.99 for every return you do. A return for two spouses counts as 2 returns.
Some 3rd party retailers may sell a special 12 return package for TurboTax Premier, so you pay one price for 12 returns. However, if you get the 12 return package, all 12 returns must be done in the same account.