I filed my taxes on turbo tax last year but this year I want to file my dad’s taxes. Can I file his on the same app

[xmasbaby0]

When you use Online TurboTax, you get one return per fee.  Each return needs its own account and user ID.   If you use the same account and user ID for a second return, you overwrite the first one and lose it forever.

https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-start-another-return-in-turbotax-onli...

 When you use Desktop TurboTax, which you install from a CD or download to your own hard drive, you can prepare and e-file multiple returns for the cost of the software.  You can e-file up to 5 Federal returns—more if you mail the additional ones.  If you have multiple returns to prepare for family members, Desktop is more cost effective.

[DoninGA]

No.  You need to create a new account with a new User ID. 

https://turbotax.intuit.com/personal-taxes/compare/online/