As you have seen, TurboTax will not split a joint return into two separate returns. If you are using the CD/Download Desktop TurboTax software, not TurboTax Online, you can use the What-If Worksheet to do an estimate of how separate returns would work out compared to a joint return. After completing your joint return, 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.
Unfortunately, switching the filing status between joint and separate is likely to cause errors in the tax return that are hard or impossible to fix. To play safe, since you have already switched, you should clear or delete the return you were working on and start over from scratch.
You should also be aware that, because of all the disadvantages that xmasbaby0 listed, it is very rare for filing separately to turn out better than filing jointly.
