Using the TurboTax online editions you can only file one federal tax return per account and user ID. Using the TurboTax desktop CD/Download editions you can create, complete and file as many tax returns as needed at no further charge.
See this TurboTax support FAQ for starting another tax return using the online editions - https://ttlc.intuit.com/questions/1894512-how-do-i-start-another-return-in-turbotax-online
