It is not easy to compare MFJ to MFS using online TT but you can do it. Since you only get one return for each account and user ID, you have to use 3 accounts and user ID’s—one for MFJ and two for each of the MFS returns. Compare, choose, and file—and pay—accordingly.
It is much easier to do this comparison using the desktop version of TT installed from a CD or downloaded to your own computer. You pay once for the software and you can prepare multiple returns easily, and it has a “what if” feature that allows comparisons.
WHAT IF…?
If you are using Desktop software:
https://ttlc.intuit.com/questions/2895312-how-can-we-compare-married-filing-jointly-with-married-fil...
If I am filing a separate return why do I have to list my spouse’s information on my return?
Even if you file separate returns (the worst way to file) you each have to list each other's SSN's and some other information on your own tax return. The IRS can then cross check to make sure you are not "double dipping" for itemized deductions, dependents, etc.
If you are in a community property state, there is more information that will be needed.
Community property states: AZ, CA, ID, LA, NV, NM, TX, WA, WI
https://ttlc.intuit.com/questions/1901162-married-filing-separately-in-community-property-states
https://turbotax.intuit.com/tax-tips/marriage/five-tax-tips-for-community-property-states/L4jG7cq7Z
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
