I need to file a tax return for someone else. Should I create another account in my name or one in His name?

You will have to create another account with his name.

Online is only good for 1 return per account.  But you can use the same email address for 5 accounts.   How to start another return in the Online version

https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-start-another-return-in-turbotax-online/00/25596

The Desktop CD/Download program can do unlimited returns.