Here's how to use the What-If Worksheet to do an estimate of how separate returns would work out compared to a joint return. This can only be done in the CD/Download Desktop TurboTax software, not In TurboTax Online.
First complete your joint return. Then open the What-If Worksheet in Forms Mode. After you open the form, before you do anything, click on Help Center. In the introductory information at the top of the Help, scroll down a bit until you see the following paragraph.
"If you are preparing a Married Filing Joint vs Married Filing Separate comparison, it is very important that you read preparing a Married Filing Joint Versus Married Filing Separate Analysis."
The underlined part of that sentence is a link. Click on that link in the Help (not here in the community) and follow the instructions there.
