No, you cannot access your return on your Dad's account when you are in a different account.  You have to be in the same account (userid & password) that you used for 2018 in order to transfer your 2018 return into your 2019 return.

The Online version you can file returns for one person (or filing joint).  If you use the same account to prepare another return, the second return overwrites the first return & you cannot get back that information.