Can I start another tax return? I'm currently preparing a joint return, but would also like to prepare a single return to compare which one would be better.

Filers using the online product must open a new account to prepare another tax return. You can use the same email address but you must have a new user ID and password. You can prepare a return just to look at without having to pay. You can't print it without paying. 

The TurboTax online editions can only be used for one tax return per account and User ID.

To start another tax return with the online editions you will need to create a new account with a new User ID - https://turbotax.intuit.com/personal-taxes/online/

Up to 5 User ID's can be associated with one email account.

To compare to Single or MFS Married filing Separate?

To compare Joint to MFS.  If you are using the Online version, do NOT change anything on your return.  You would have to start with a new account and do a test return.  You don't have to pay unless you want to print it out.  So you might need 3 accounts, one for Joint and two MFS, one for each spouse.

How to Compare Joint to Married Filing Separately

https://ttlc.intuit.com/community/married/help/how-can-we-compare-married-filing-jointly-with-married-filing-separately/00/26551

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

It is usually better to file Joint. Joint has the lowest tax rates and the highest Standard Deduction.   And if you are in a Community Property state MFS gets tricky to figure out.  Here's some things to consider about filing separately……

In the first place you each have to file a separate return, so that's two returns.  And if you are using the Online version that means using 2 accounts and paying the fees twice.  

Many people think they come out better when filing Married Filing Separate but they are probably doing it wrong.  If one person itemizes deductions then the other one must itemize too, even if it's less than the standard deduction, even if it is ZERO!  

And there are several credits you can't take when filing separately, like the

EITC Earned Income Tax Credit

Child Care Credit

Educational Deductions and Credits

And contributions to IRA and ROTH IRA are limited when you file MFS.

Also if you file Married Filing Separately up to 85`% of your Social Security becomes taxable right away even with zero other income.

See …….

https://ttlc.intuit.com/community/married/help/is-it-better-for-a-married-couple-to-file-jointly-or-separately/00/25590

@orlandosoco wrote:
"I'm currently preparing a joint return, but would also like to prepare a single return to compare which one would be better."

If you are married you cannot file as single. If you are not married you cannot file a joint return. Did you mean married filing separately? If so, see VolvoGirl's reply above.