Can I file two separate tax returns with one Intuit account? Or do I have to create a separate account for the other party's return?

[KrisD15]

No.  

When using TurboTax Online, you cannot use the same account for more than one Taxpayer. 

You can use the same email for the other account (up to 5) but the password must be different. 

TurboTax Desktop can compute more than one return, but you must close the first one and start a new return. 

[VolvoGirl]

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-onli...

Or go to the IRS Free File programs

Free File: Do your Federal Taxes for Free | Internal Revenue Service

The Desktop CD/Download program can do unlimited returns.